I install an 2.50" catback whit 3.5" pipes, and I fell that the car runīs less then when I have the factory exhaust !! Itīs better the stock exhaust or what ?? 2004 mustang GT, 4.6

 

You might be feeling a SLIGHT decrease in low end torque. But its probably all in your head. Catbacks dont really effect power at all.

 

3.5 a bit much

 

the 3.5" are just the tips. The rest of the system and the only part that means a damn is the skinniest bit, which you've said is 2.5" on the catback. It's 2.25" on the stock H pipe so all you've done to power is change the sound it makes. I'd be shocked if you changed by 1% rwtq or rwhp.