2.5" is fine, 2.75" is shady, 3" is more than likely to cause a loss in performance.

Don't eliminate your cat - we have enough trouble with black eyes from the enviroMENTALists as it is! Besides, a catalytic converter also functions as a spark arrestor offroad -a muffler does not. A high-flow cat won't cause performance difficulty, and some high-flow cats have actually been shown to improve exhaust gas flow (cf. Random Technologies.)

The upstream HEGO (nearest the engine) is there to monitor fuel trim and allow for realtime adjustment.

The downstream HEGO (after the cat) is there to monitor catalyst function. I'm unsure how the signals are interpreted, although I have seen "simulators" for deleting the downstream HEGO without throwing a code - check speed shops (Jeg's and Summit Racing leap readily to mind.) Note that this can get you downchecked for emissions - easier to have a bung welded in the pipe after the cat, if you have to.

But, keep the cat. However, if you're having a header made and have the option, find precats that don't cost too much and have the header made with flanges so you can easily replace the units. Overall, it ends up being cheaper.

(When you are designing a "clean sheet" setup, design it and build with the guiding principle being that the next poor schmuck to work on it will be you - you'll have a Swiss Army Knife, a rock, and a book of matches. Design and build accordingly.)

EDIT - Oh, and avoid sudden changes in pipe section, since those can cause backpressure and flow turbulence as well! Section changes should be smooth - no ledges - and should happen at 3:1 or smoother. That means that a change in diameter of 1/4" should happen in no less than 3/4" of pipe length - and longer is better (1/4" in 3" would be better, but that sort of ratio can get cumbersome and space-consuming...)

 

5-90 Nailed it.

I have the Cali emissions package, so I have 4 O2s at the manifold. Here in SC I could have cut the cat off but I'm a tree-hugger and kept it; only planet we have and performance gain is minimal in real world driving. I kept the 2.5" downpipe and stepped from 2.5" to 2.25" between the cat and muffler and kept the 2.25" all the way out to my tip. All stainless.

Also, I'll try to take a video and sound level for you so you know how my set up sounds. It's really only loud with the windows down and under acceleration.

 

Here you go. A little wind noise but it was my digital camera.

http://youtu.be/DwzIp2HOY-o

 

I went 3 1/2 dual inch cat back now I have back firing in exhaust while driving low rpms think this is because the large pipe????

 

That is highly probable. That is WAY too much pipe size. And what is "dual inch?"

 

guarentee its not a true dual. you still have 1 header that comes off the motor into a collector. Its mostly likely just 2 tailpipes, which does absoulutly nothing.

 

sorry should have been more detailed, its not true dual splits at rear axle