first off, I did not feel like I was anywhere close to the top weight on the medium Burn. I guess I would say I'm somewhere in the middle. I would be happy to put 35lbs of overnight gear in it and still creek. it was the h3 245 that felt like I was at the top of the weight range at 170.
as for comparisons to the M3, I would say that the Burn is certainly higher performance. I haven't paddled the M3 in a couple of years, but I would say the Burn is a good bit faster than the small one, and at least as fast as the big M3. it's also narrower, which made it easier to get good aggressive vertical strokes in. it planes out better than M3 so it ends up turning faster and being very maneuverable at speed.
I wish I could say more, but I would have to go back and paddle the M3 again.
Dave - good hearing from you as well. Hercules looks like a solid boat - should be a good one for you.