Is an 18in barrel too short?

[belly-deep]

My one concern is that the 7-08 is not a heavy hitter compared to the mags or 30's so is that loss in fps a bigger deal with the 7-08.

Run some numbers through a ballistic program and see what turns up. Generally, fast bullets slow down fast too. In other words, a 100fps advantage at the muzzle might be less of an advantage at 400 yards.