personally have no problem keeping the cat on my ZXT - when I went to 3", I did a 3" cat too. I may lose 1-2 HP, but feel better about not polluting as much.
Of course in order to get registered in the Metro Atlanta area, you have to have a cat, but I'd do it anyway.
I read somewhere that in the case of the LT1 (and probably the LS1/6) engines, that the use of EGR allowed advanced timing and better fuel mileage while cruising. Under full throttle applications, the EGR didn't work. To my mind, with (factory) computer controlled cars, you don't lose much and in fact gain a lot with the standard emission controls.
I'm not saying they can't be improved on for better performance (tweaking ignition and fuel maps), but don't think the trade-offs in removing emissions is worth it.
JMOHO