With the intent of increaseing low to mid level torque, would it be better to get a high flow cat or performance muffler, or both?

 

a high flow cat should be ok, I have a 3" in/ dual 2.5" out flowmaster 50 series and that worked pretty well once I downsized the 2.5's down to 2.25. that brought back a lot of low end torque and is equal to a single 3" in/out muffler. if you go too big on the output's you'll loose some low end torque, the same thing goes for the intake side... open it up too much and you'll loose some low end.