AS you say difficult to judge but 615hp on mufflers? That ain't shabby at all. Assuming 180 degree header arrangement on megs, I am not sure that, say 6,000 rpm these motors would sound that different. What I am trying to pin down is whether sound is dictated by swept or stroked volume and cam choice rather than pure CC's. I guess my engine guru hero JacMac should chime in now as he will know.
And I'll bet he will be watching this as I write.
If I was in the game now, I would definitely have Jac build me an engine and insist he flew over to adjust the tappets between rounds!!