Looking at my Y pipe/cats (Dynatech stainless), there is no easy way to replace the cats with straight pipe. Each cat has a flange on the end, and they are set at a certain angle to match up with the header. Also one cat has a bend right after the cat, so clamping or even welding a pipe onto it would probably mess up the alignment of the other end of the pipe (Y pipe is 3 sections clamped together). So unless there was an exhaust shop to actually fab something up, its not going to happen - and I dont know of an exhaust shop around here that is willing to touch the cats.

So...this might be a stupid idea, but what if I were to take a ~2.5" hole saw and just run a hole straight through the cat (inside is stainless 'mesh')? Would this cause more turbulance than flow?

Here are some pics of the cats:

 

depending on how good you are with a welder I dont think replacing the pipe with straight pipe would be hard, just measure very closely. If you dont want to do that then see if you can find an extension to get a holesaw that far in there then ram a bigger piece of pipe in there to get all the rest of the crap out of there.