Your bullet logic is...well, illogical. Missiles fired from a plane going faster go faster because they are propelled. Bullets fire from a plane going faster hit higher resistance and slow immediately, so neither muzzle velocity nor terminal bullet speed change because friction and gravity are always slowing a bullet down.
6x.50's should be able to tear up light armor vehicles and soft targets like trucks. In real life a .50 passing close to a human is enough to disable if not outright kill a human. I think people get wound up on effectiveness of the .50 though; while it is a high velocity, high energy round it's still a small caliber bullet. It has no mass really, so for it to be effective you have to put a shitload of rounds in target AND hit your target at convergence. If you aren't massing your bullets you aren't getting your all out of a set of .50's. That was the magic of that weapons system, when you hit on convergence you have the energy and the bullet mass to do real work.
I'm sure a lot of you have time behind the M2 like I do. It's an impressive weapon in many regards!!