My solution for that was an exhaust dump that I made installed before the cat. Basically it is a Y pipe, one route goes throught the standard exhaust (feds would be happy) and the other, when uncapped, would vent directly to the atmosphere (for race purposes only [
]). I've got pictures of the setup on my other car if anyone is interested. The results were that the boost spooled up MUCH quicker and gained quite a bit of top end power. It wasn't really all that loud because the turbo acts as a muffler.