Stock Cat Specs vs Y-Pipe
 

Does anyone know for sure what the inlet and outlet sizes are for the stock cats and y pipe?

Seems like some aftermarket HFC's are 2.35" and others are 2.5". How much of an effect on flow would having a switch from 2.35" cats to 2.5" y-pipe be, with regard to turbulence generated by the change in size? And if our stock cats are 2.35" would it be better to mate that to a Y pipe that is 2.35" inlet?

EDIT: Searching lead me to stepped exhaust theory which then lead to this: http://my350z.com/forum/intake-exhau...k-for-n-a.html
Halfway down, very informative, but I'm not sure whether it would apply to my situation.