gains from y-pipe back exhaust????

[njspder]

hey gang,
i am gonna be helpin griffis3 with his exhaust soon. we are lookin to do a new y-pipe using a flowmaster (2) 2.5 into a single 3 inch system. the cat will be eliminated(O2 simm will be used), the cat will be replaced by a 12 inch dynamax bullet muffler welded into an old cat shell i have. the system will have a 3 inch s-pipe and after cat leading to a flowmaster 80series.
anyone do anything similar to this or have any idea of what kinds of et/mph gains can be expected from this kind of exhaust work?

thanks
tim

[ReodDai]

hmm you probably should have put this in the EXHAUST section....