The only real way to get a performance gain from a main cat is to gut it and prefurably replace it with a peice of pipe. Like you would when removing the resonater. Hince why most, if where planning on gutting or have gutted their main cat, will go with an off road y-pipe. This off road y-pipe basically replaces your current hellbent y-pipe with one a little more flow friendly and no main cat. You can get the y-pipe with a high flow cat, but I am not sure how many have one or the other.
Now some will tell you that a y-pipe can actually hurt you in some ways. There were some mounting issues along with noise (vibration), but those have pretty much been taken care of. Also with heat. There are some concerns among a few here that without the stock y-pipes heatsheild, which basically protects the lower engine from the awsome amount of heat that can be produced from this area, you are asking for trouble. This is addressed by getting one with a ceramic coating (or you could paint it yourself) and can be further addressed with heat wrap.
Overall though, you would not gain a whole lot from just adding a high flow main cat. Notice I said, "Not a whole lot!". You might gain more by just taking it out of the picture.
As for the precats, either gut them or get headers.
