Quick follow-up: I just got off the phone with one of the Hornaday Tech Reps, and asked about the new .357 magnum 130 gr MonoFlex Handgun Hunter compared to the 158 gr Custom XTP; essentially from our conversation it sounds like it's a solid copper bullet option for areas that do not allow lead bullets, and that the 130 gr GMX bullet used (solid copper with an elastomer plug in the hollow point) will provide equivalent terminal performance to the 158 gr Custom XTP, with particularly good through-and-through penetration performance.
Detailed technical/performance information on the Handgun Hunter cartridges Hornaday website is currently sketchy, as their R&D hasn't released their results yet, at least for publication; the rep said they'll likely be out within the next month to month and a half.
Best, Jon