Datalog it, watch the O2's, but NB O2's are NOT trustworthy for jack <>0.45v.
You really need a WB on there to know. I don't know how much extra bandwidth in the stock injectors on the 3.8L Turbo, but on my 3800, there is NOT enough, and it does go lean. With 100% E85, mine went lean around 14:1 under boost, >100 mph, not a good time to go lean.
With 50% E85, it isn't as critical, likely won't go as lean, but still, if you don't have a lot of bandwidth already, aka, mods that need more fuel than stock, you shouldn't play with it until you have a way to know what the actual A/F is.
Holes in pistons just plain suck.... And I would bet your EGT's are SOARING running a mix of E85 and not a chip.