I checked and it is a plastic tank. Since swapping the power source of my rewire from the fuse box to the alternator it seems to have helped a bit. Now at WOT, the car is a low-11's AFR until about 5500rpm, and then slowly creeps up to a low 12's AFR (12.2 max at shift point). I'm going to replace the filter just for kicks. One thing I noticed is that my LTFT's are now mostly in the negative. When cruising they go as low as a -17.1, but then back to 0 at WOT. I was told they should be +/- 10 at the most, so maybe that has something to do with it.
Regardless, I have heard of guys running mid 12's for an AFR with E85, so I think it can still be tuned well enough.