The high flow cat is'nt any louder than stock as yours sounds much like mine. I'll be dropping the cat and replacing with a straight 3" pipe in a month or so. I'll try for a before and after sound clip to compare.
It'* not that much louder without the Cat....wish I had a recorder and video cam....but the sweet sound of the headers is less muffled.
I connected my 3" off road pipe just before the stock resonator just before the O2 sensor coupling where the pipe is 2 1/2". After the resonator, the pipe reduces down to 2 1/4". I will eventually go 3" all the way back and split into twin 2 1/2" in and out Borla style mufflers.
Note that you will not find a 3" to 2 1/2" adapter but a good shop will be able to swage the two pipes and fill it up with weld.
Also, I couldn't find a proper plug for the after Cat O2 sensor, so I just left it in even though it is not connected to the wire harness.