I got the MBRP downpipe back, liked it alot. Just recently put the cat back on for warranty. The cat definately makes it quieter. I then got rid of the muffler completely, running straight pipe from the cat back. I cannot tell any difference between the cat back muffler system and the cat back straight pipe. Both power and sound (loudness) seem to be the same. I am leaving the muffler out. Good luck!