who cares about a cat converter. get a jdm type r header and cat if u like. thatl solve the problem there. 60mm is a good size for exhaust systems on n/a cars. its 2 and 3/8 inch i believe in us terms. thats what most aftermarket companies do for hondas such as greddy, apexi, hks, etc... youll probably gain more power with the jdm r header/cat than you will with an exhaust system. IMO do both at the same time to get the thing to breathe. i would go no bigger than 2.5" and no smaller than 2.25" . i belive 60mm is right in the middle. thats what im runnin on my ls vtec setup. the bigger motors like the k20/24 and h22/f22 could prob stand larger piping due to the bigger displacement. hope this helped.