I have tuned many many many V7s v8s and v8 twin scrolls. They make around the same power. The Twin scroll does have a slightly wider power band on the TRQ curve.
The Power difference in V7 and V8 single strong is simple. The V7 cams are weak. The V7 RA has the better cams and the V8's came with the V7 RA cam style. So off the bat, The V8 is going to be a bit better then the standard V7.