You might get 4 or 5HP across the board but don't think 10 or 11 because you will not get that. There is not one good dyno sheet that proves that yet, and I don't mean some spike when the clutch shifts. I pulled the cat and resonator off and could feel a difference in the mid range for sure, since the only dyno is 250 miles away I was not going to drive 500 miles to dyno it. So the question is, is 4 or 5HP worth 1000.00 to you? you may be money better spent putting a clutch in.
you shouldn't need a dyno to tell you that if you install an aftermarket exhaust and you have to add fuel, you are going to make more power. The question is, is it usable power and are your clutches tuned to take advantage of that power…..So simply replacing the exhaust may actually lose power unless you compensate for the extra airflow with added fuel and that added power is in the useful part of the powerband and you are clutched correctly to utilize that extra power. Most dyno sheets either show just the before and after numbers after switching out the exhaust and a few show it with a the fuel controller added as well. I don't think I have seen any that have done a stock run followed by a run with exhaust, fuel controller and clutching….