very good numbers!
on my 97 i hollowed out the stock cat (for now), took it off the back of the stock cat with a 3" into 2.5" Y and ran dual 2.5" all the way out the tips with cutdown and repacked 10" cherrybomb glasspacks. with you still having bullet cats but eliminating the front Y i would say i should have gained at least that much. my ultimate plan is to make it duals with an X section by eliminating the stock crossover pipe. true 2.5" diameter the entire way through the system...should be sweet when finished...
but you are right. our floorpans are vastly different but it wont fit in the tunnel if you run dual 2.5" the entire way. mine sits wedged in there so it doesnt squeak or rattle too badly. the front Y once it's gone should provide the room i need for the X section and i dont count on having to modify things too much more with the exception of remaking the 2 underbody braces that go across the tunnel on my car. i will need to make new ones that hug the floorpan so i can make regular exhaust hangers...