I find that it's reasonably easy to shoot a 90 with most any ammo. Once you begin to push that, though, the ammo makes a big difference. The S&B stuff is one of the better ball loads in my experience along with 115gr Fiocci. 95 and better requires good ammo. Personally I have yet to shoot a 100. However, I can pretty much pull 95+ on-demand. At this level of performance, there are 2 key factors that come into play:
1) Visual Focus: Your sight alignment has to be spot on. Front sight correctly aligned to the rear (equal height/equal light) and sight alignment on target. Then, hard front-sight focus through the trigger pull. If you let this slip, those 8's and 7's start to show up pretty quickly.
2) Neutral Grip: I still grip the pistol firmly, but the pressure has to be applied correctly. 25yds magnifies flaws that don't show up 10 yds and closer. This was my first big "AHA" moment in 25 yd shooting and it happened literally in one day. For me, rolling the support hand way forward ala Vogel created more opportunity for torquing the pistol and thus significantly shifting the POI based on varying grip pressures. I found that by rolling my hand slightly rearward, I was able to apply ample clamp pressure with my support hand while keeping varying pressures from disturbing the sights. Diagnose this with a dry gun by taking a sight picture with the gun loose in your hands and then applying your normal pressure. If you have to apply counter-pressure or re-align your sights, something's off with your grip.
As far as the hardware itself, I find that matters less and less. Sights for instance. HD's are huge, but you can still do good work with them. You just have to figure out what it is you need to see in terms of alignment and then duplicate it every time. I find that with HD's if I hold the tops of the blades on the tops of the black of a B8, I get center hits as long as I do my part. See below of a Gen 3 19 I traded into with HD's already on it. I hate the Gen 3 19 because of the hump (much improved on the Gen 4), so likely my misses here are more due to grip issues, but the sights still allow good hits. I also want to say I agree with you that maintaining a shooting rhythm can play a factor here; especially as it relates to consistent grip and sight picture. However, as you develop the grip and sight focus, you should reach the place where you can duplicate them correctly even if you re-holstered between shots.
Hope this helps and keep up the good work!