So if you 'lost' around 7hp with a chambered exhaust and stock exhaust manifolds, would you expect to regain those 7hp with chambered exhaust and long tube headers?
Also, would imagine that in reality, there could be at least a 7hp variance between 2 otherwise identical (drivetrain) cars off the factory assembly line.
Ding ding ding!!
With my experience of putting countless late model LT & LS cars on a chassis dyno some cars put down a few extra ponies than another. Same setup put it boils down to tolerance stackup. You have mins & maxs and it you have the incorrect balance then you will have the lower output rate. Same can be said about the opposite. The fact is you are not going to feel 7 or 10 or even 15hp by the ole "buttometer".. Even the calibrated ones..
If you are not competitively racing then its all about sound. If you like the sound of chambered then install it. If not then sell it.