Got it solved.
You'll never guess what it was either.
OK. So I replaced the positive battery cable, improved the grounds and checked all the wiring. Same issue, though voltage was more stable. It still cut out WOT high RPM.
For some reason I played with the gauge cluster and ended up finding out somethings wrong with it. I swapped in the cluster from the other car and it works perfect. So the gauge cluster didn't dim anymore. The headlights still did though and logs showed a big voltage drop under the same conditions.
So today I'm going over what I can change in the tune and I come across something for the high speed fan under high load. I never noticed it before.
So I figure WTH, I'll adjust everything up and see what happens. High enough that it shouldn't kick in under heavy load ever.
And now voltage is good across the board. Weird.
I do think the alternator is a bit weak though. But this should prolong it somewhat.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.