U23 MMR Conclusion
After trying it, it looks like adding MMR was a mistake.
The Problem That Was Faced: Heavy armor builds aren't attractive enough, because they prevent use of character features that reduce damage by a percent on a successful Reflex save, and by a different percent on a failed Reflex save.
A Straightforward Solution: Create some character features (in existing feats and new enhancements) which allow heavy armored characters to reduce damage by a percent on a successful Reflex save, and by a different percent on a failed Reflex save. (This aligns closely with the existing character feature, and doesn't stack with it, so it's easy to make them balanced)
A Complicated Non-Solution: Create a whole new stat, available from multiple new sources, which reduces damage by a rating and applies to most things with Reflex saves, and also lots of things without Reflex saves. And which doesn't work on some Reflex saves. Allow it to work in conjunction with existing percentage Reflex-damage reducers, but with stat sources being lessened according to armor category. (This works on a much broader set of effects than the old feature, and can stack with it, so it's hard to make them balanced).
Is it completely impossible to adjust MMR so it works acceptably? No, with enough effort and special-case rules it can be tweaked to fit. But it's not there yet, and the other approach would've been easier and more reliable. Right now Reflex-dump characters are too protected against Reflex attacks, characters in general are too protected against many non-Reflex attacks, and legacy energy defenses are integrated poorly. Wearing a big steel suit prevents Lighting Bolts, hurray.