There are a lot of things that you are allowed to do in E Street Prepared, but honestly the most valuable for me is the ability to backdate parts off of any car on my line from Appendix E of the 2014 SCCA SOLO rulebook (my line just says All S197 including Laguna Seca, however, a 2005 gt could theoretically backdate a Boss motor and transmission in, pick up 140 horsepower, and have a huge weight advantage over a factory boss...)
Other than that, I can make a custom splitter and rear spoiler (both are in the works, a hybrid of fiber, Kevlar, carbon, and vertically oriented cardboard for a core material, and of course a second trunk lid, so that I am not stuck daily driving with that), DOT approved racing slicks of any size (I am enjoying 315/30ZR-19 front and 295/30ZR-19 rears on Hoosier A6's, engine work outside the engine (no forced induction, unless factory equipped, but ported manifolds, headers, cat deletes are allowed), Watts link in conjunction with a Torsen T2R make the car drive like a big go cart.