I can answer a couple of your questions.
Brightness at 100% is comparable to other batteries, the output of course, depending on how you've programmed it. In the field there's simply no functional difference between, say, 70% and 100% so the control you gain with the Core is welcome. BTW, alkalines are the poorest choice for the stock XP2—NiMH perform better, as do disposable lithiums.
High and low modes are programmed separately within wide output ranges, so two modes are retained using the Core. Red mode is not programmable but is also retained.
Hope this helps,