Doesn't seem like even a straight pipe on a R3 makes the same ruckus at idle that a hog does. I was just in Port Dover on Friday for their Friday the 13th "rally" and not even Carpenter's thunderous Brute idles with the same kind of volume these guys were packing. As far as I'm concerned, a loud exhaust should be a symptom of a high performance build, not a replacement. I'd much rather be fast and quiet than slow and noisy.
As for the OP's original point (as I took it) I agree that there is a resonance and menace to an open piped Harley that SEEMS like should be present based on the size of the pistons on the rocket. Previous posts appear to explain some of the difference, but I'm not going to complain.
Hogs do sound good, but when I rev my big girl's engine the earth not only trembles beneath her, it travels beneath her... and fast.