Actually the 7mm, especially the more potent versions get down range and hold velocity very well, the thing is, they dont buck wind quite as well and dont arrive with as much authority as the larger calibers.
At 600 yds, it's a descent round, however... I would still take a 300 WSM
over my old 7mm RM at any range. It will push a 180 gr bullet about the same velocity as a 7mm does a 160 with just a few more gr of powder and be a little more effective.