IIRC stock turbos are at their peak efficiency around 18psi. Going past that with E85 your IAT are still "acceptable", but I'm betting you are over speeding the turbo & creating unwanted back pressure. With the rear runners being 1" long or shorter on any cast manifold option, higher back pressure will result in exhaust gas revision into the rear cylinders. But the turbo will continue to make more power past 18psi....