Vibrant makes some very nice cats, they are however not drop in.
Magnaflow makes nice metal core cats also. Again not plug and play.
Tru bends does list stock replacement high flow cats.
I have kooks cats with my setup and I'm very happy with them.
Do the mallets have a stock exhaust or long tubes? Either way you are most likely going to be welding in a set.
Best advice is get a 200 cell if you need max performance and noise, a 400 cell for less noise and minimal impact on performance.