[texan]

The turbo does significantly deaden sound waves from the exhaust, but depending on the airflow it'd still be quite to very loud with no muffling or baffling at all. Moreover, bypassing the catylitic is against the law in all states when driving on public roads, but of course that's not an issue at the track.

If you want to create a bypass valve I recommend you have a local muffler shop weld in a dropout just upstream of the cat, which is opened by removing a metal plate from the end. It's very cheap to do... you just need a 2" section of pipe the same diameter as the current downpipe with a mounting flange welded to the end, along with a similarly drilled metal plate complrete with gasket that mates to the flange.

Of course the real answer here would be to sort out your exhaust system so that it's no longer a real restriction, that way you'd have nearly all the power of a dropout exhaust all the time, and without any of the noise, safety or legal ramifications of an open dropout.