How much HP would be lost by using the stock cat with no resonator on a 2.5" custom exhaust with 4-2-1 headers compared to a high flow cat and a resonator?

 

There is not way to tell unless you do a dyno test with the stock cat, then replace it with the high flow cat.

I would venture to guess something on the order of 1 hp minimum to a 5 HP maximum. The inside of the stock cat is rather small (inner diameter) and it would create a fairly significat amount of backpressure.

 

Do not get 2.5" exhaust if you have NA car. I tried both 2.5" and 2.25"... although I did not dynoed it, I drove car with both setups long enough to tell the definite difference. When my car had 2.5" exhaust, its engine response time plus acceleration was REALLY slow. This was especially noticeable when I drive at 50MPH, let the gas pedal go off freely for a second, then press gas pedal hard. With 2.5", it took REALLY LONG time to get throttle response... on the other hand, 2.25" exhaust gave me better engine response.