T5superduty

Which is better 2.5 or 3"

MelloYello

Flowmaster just came out with a 3 and a quarter exhaust or half cant remember!! Id go with 3......

vfast

3" system won't help you unless you have bigger primary tubes on the headers

I would do the 2.5 and have the 3" tail pipes
the exhaust is tuneable to change the rpm range so bigger isn't always better..I can tune a set of header with 2.5 that will just kill a 3" exhaust...this is done with lengthing the collector tubes on a long tube headers....
if using a 3" exhaust
2" primary tube headers would be needed and you bottom end would suffer unless a tubro or S/C is added inorder to benifit the engine would needs to be able to spin 7-7500 freely so your heads will be the restriction...and with 3" exhaust cam shaft overlap needs to have some attention

T5superduty

Well they are 1 5/8

vfast

then I would go with the 2.5" and maybe the 3" tail pipes for a nice look and a lil better flow if and when you upgrade again