One of the exceptionally nice features of many contemporary HK handguns is how their grip can be significantly tailored to an individual/individual's hands. In my experience, this provides for both accuracy, comfort and index, and can be an excellent means of providing ergonomic integration with the overall pistol.
In my experience with my VP40 and P30L, the key portion of the equation is the initial backstrap selection. HK provides significantly differentiated backstrap profiles, with the differences being in both size and hump placement/presence.
Participants on the HKPro site provided the best backstrap selection guide (HK provides the pistols with the Medium choices installed) : "While leaving the side panels alone, swap out the different backstraps, while trying the following: Look at a target object with the pistol at your chest prior to press-out, close your eyes, and while keeping them closed, press out the pistol to your natural shooting position. Open your eyes and observe where the front sight is vertically. If the front sight is too low, install a larger backstrap and repeat the process. If the sight is too low, install a smaller backstrap and repeat the test. Ultimately, you want the backstrap selection that provides front and rear sight alignment on target naturally on press-out."
That selection process works superbly, in my experience with both HK and Glock pistols. While it requires that the roll pin securing the backstrap be removed, using a correct sized punch facilitates this with little drama, and you really don't need to hammer it out, firm hand pressure normally suffices.
After determining what I feel is the appropriate backstrap selection, I proceed to verify with live fire, both as a zeroing process and in dynamic shooting.
Side panel selection is a bit more nebulous. The one quantifiably discernible portion of the side panel selection equation seems to be regarding lateral accuracy; if the selection "pushes" groups to one side of the target, that tends to indicate that a smaller side panel is desirable, with the key panel being the one opposite the side of the group drift. In other words, if a group is to the left of the target, a smaller Right panel size is indicated. However, beyond achieving natural mechanical accuracy, comfort may be a factor as well, as can subtle differences in respective HK pistol grip area contours.
What I have found is that in addition to mechanical accuracy, the right backstrap and side panel selection serves to seemingly automatically lock my hand in and facilitate my index to the gun very naturally, providing a very fast, natural/instinctive sight and target acquisition on draw and presentation. It can also aid in trigger pull and control of both the initial and subsequent shots.
I mentioned differences in HK pistol grip area contours. In my case, that involves my VP40 and P30L. Wile the VP receiver is very similar to the P30's (and was in fact designed off it), their are subtle differences in the rear tang areas between the two; the VP has a slightly deeper tang, enabling the hand to come up higher and perhaps slightly deeper on the receiver. In practical terms, this means that you may well encounter differences in backstrap and side panel selections with different HK platforms.
In my case, the backstrap selection for both was the Small one-although it was a much more crucial factor with the P30L. While I could fire the VP40 with the OEM fitted Medium backstrap adequately, it was better and more natural, and more accurate (for POI equaling POA) with the Small backstrap; but with the P30L, I found myself fighting the gun with the OEM Medium backstrap-the Small one was absolutely crucial, both from a results and comfort standpoint.
Side panel selection was a bit more of an empirical journey. Initially, after much research (the HKPro forum and Todd's extended P30 test on pistol-training.com provided the best inputs), I went with Small left and right panels, and then switched the left to a Medium (I'm right-handed); Todd ended up with this selection on his P30; initially he was wary that the Medium left panel might constrain his access to the left slide release, but he found that such was not the case, and preferred the Medium left for his support hand. I ran with this selection on both my VP and P30 for about a year. I then decided to re-look the entire backstrap and side-panel matrix. I quickly re-confirmed that the Small backstrap was in fact the ideal selection for me (both the Medium and Large backstraps did not provide the best index and instinctive sight picture/POA/POI results, and the larger (especially the Large backstrap) back strap's hump were very uncomfortable in feel-the hump too aggressively protruded into my palm. If it/they provided improved accuracy, I would have tolerated it (keeping im mind Clint Smith's dictum that "Handguns should be comforting, not necessarily comfortable")-but they didn't, so the continued use of the Small backstrap was a no-brainer.
On the P30L, I found, somewhat to my surprise, that use of the Large left and right side panels provided for a more comfortable, hand-filling and control experience. On the VP40, I found that the Large panel worked well for the left, but the Large right panel tended to push my groups to the left; switching to a Medium right panel provided both center hold nicely centered POA/POI, along with adequately and comfortably filling my string hand palm. I think that the subtle receiver differentiation between the two platforms led to the differing right side panel selections.
When I quantified the HKPro selections among the 39-odd thread parcipants, the most prevalent selections were for Mediums all around (the OEM HK setup), with the remainder pretty much equally split between Large and Small backstraps, and those not choosing the Medium right panel tending to select the Large panel, and those not choosing the Medium left panel tending to opt for the Small panel.
Interestingly, in a March 2013 Gun Tests review of the P30L, they spoke with HK Shooting Team Captain Jason Koon, who specified that his normal preference is for the Medium backstrap, Large left panel and Small right panel. Again, Todd's ultimate selection on his P30 was going with a Small backstrap, Small right panel and Medium left panel.
So-What does all this mean??? Well, I definitely recommend using the HKPro site's selection protocol for the backstrap selection, followed by empirical live-fire testing and verification. For the side panels, I'd probably start with the mediums, and adjust as necessary for windage fall of shot patterns. Accuracy, trigger finger placement and palm fit/control is crucial in side panel selection; don't be unduly surprised if asymmetrical panel selections provide the best results, and don't necessarily assume that works best for you on one HK platform will automatically be the recipe for other HK platforms. I also suspect that differing HK action selections/triggerpull weights between various HKs may dictate different selections as well.
While ultimately it's a highly individual selection process, I'd be very interested to see how and what other p-f HK users came up with as their ideal setup, and what they experienced en-route.
Best, Jon