The cat really doesn't restrict flow but, it will make your EGT's a little higher with it than without it. I noticed an honest 75 degree drop towing the same trailer up the same grade with out the cat. To some that may not be enough to remove it but, if you are getting close to that magic max EGT it helps. For the muffler just try and make sure you get a flow through muffler. Easy to spot if you can look through it then you have a flow through.

LT.

 

Grab a peice of pipe and have it welded on in place of your muffler. Genos garage used to sell a straight pipe kit for $90 and when I had my pipe welded on it cost me $45, and it sounds great, for the price. I got my pipe for free and lucked out though!
Scott

 

I dont know about you all, but i noticed more taking the cat off than i did taking the muffler off.. I took the cat off first, ran it for about a month, and then took the muffler off.. just taking the cat off, i honestly picked up a couple handcalculated MPG...

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but as others have said, the straight pipe was too loud for me, and then when i got 5 tickets in one night and one of them being a loud muffler, i got a muffler out of a silverline exhaust kit a buddy was putting on his 2nd gen and was going straight pipe, and i like the way it sounds.. I wish i could find a magnaflow muffler. The magnaflow makes these truck sound good..