I used to have 3" H pipe exhaust, off of 3" collectors. At the time the car ran the same mph with or without the exhaust. Mufflers at the time were Hooker Aero Chamber.

Now I have 3.5" X pipe exhaust, coming off 3.5" collectors. Mufflers are Dynomax. Haven't bothered to test with/without.

Maybe both are overkill for my current power level...

I may never find out as I don't get too many chances to run with open headers.