I have the version with the 16" heavy/straight-sided barrel. A buddy of mine has the same rifle, except his barrel is the M4/military profile. Both have similar numbers of rounds through them, and no modifications. We shot next to each other at the range a while back, then traded rifles. I noticed no difference in my groups at 100 yards with iron sights, and neither did he. (Obviously the groups were in a different place, but no change in size.) My opinion is that the accuracy difference is immaterial at the distances most people shoot unmodified Bushmaster rifles.
I am REALLY pleased with how this rifle shoots. I can do 3" groups at 100 yards with iron sights off a bench, no problem. Two inch groups if I take my time.
I suspect that the M4/military profile will heat up and cool off quicker, since there's less metal. Perhaps that means it's not as strong, but as long as you're not prying open ammo crates with your barrel you should be fine.