Thanks for the shifter pics; that gives me a good idea of where it's all going to come through. Might be a challenge to get a center console to work with that in my application; it almost either needs to be further forward or further back, and you've already got the rear tower relocation on yours. I'll have to put some more thought into that.
You might try disconnecting the rear swaybar (pull one link and zip tie it up out of the way) and see if that helps with grip in the rear. It's something that's done a lot for me on overpowered autocross cars in the past where they would otherwise open the diff up or oversteer badly. Yes, it will give you more understeer mid-corner, but with a little practice you can get around it with trail braking on the way in and focusing on apexing later. And it's a cheap, easy thing to try.
If your rear shocks are adjustable, you can also try softening the rebound in the rear (or to a lesser degree due to how flexible our frames are, stiffen it in the front) to bias the transitional handling towards understeer which may help make it feel more predictable.