I dont see the exhaust being the choking point, particularly if its way back at the muffler can. On e85 you should be far more HP then me if we were on the same dyno?!? I was on a Dynapack near sea level at 85+ degrees
I have a similar setup with the older non HTA Green 850cc FMIC and at 20psi im at 391hp on 93oct.
Only thing I can think of is to check the gaskets on the TGV to spacers. I have seen them stick near an 1/8" into the air flow path, just because they are too wide. Also check the transitions from Manifold to TGV to Spacer to Heads....the smoother the transition the better.
Next time your on the Dyno take a laser temp sensor and shoot the TMIC and see what the core is at during the pulls. If its ridiculously hot that will not help either.