If you are set on doing it, then why not rent some dyno time and test what you have? Then change the exhaust and go back to the dyno. Then you will KNOW what happened.
I'm not the only guy I know who has never found power with this kind of thing. And merge collectors which were all the rage years ago. You have to think from the air intake (scoop or air cleaner) all the way to the end of the exhaust, including what intake and exhaust lobe you use, even the size of the exhaust port and valve. The real power from stuff like that is to be found in the intake side of it.
Likely you will find as a best case scenario that you won't find anything except a lighter wallet. If you don't test, you *** dyno is no better than the next guys *** dyno.