I was reviewing the Winchester ballistics tables for the .308, .30-06 and 300 WSM and then came back and found this. You're right, the .308 does start to gain a little ground at 500 yards, although, the gains aren't really very big.

At the muzzle, the .308 is 100 fps slower than the 06, and 475 fps slower than the WSM. At 500 yards, the .308 is only 7p fps slower than the 06 and 375 fps slower than the WSM. In energy, this translates to 191 ft lbs less than the 06 at the muzzle and 968 ft lbs less than the WSM at the muzzle. At 500 yards, the .308 has 100 ft lbs less than the 06 and 513 less than the WSM.

These comparisons were all with the 150 grain XP3 premium ammo.

Comparing the .308 to the 7mm-08 (using a 140 grain ballistic silvertip), the 7mm is 55 fps slower at the muzzle and 11 fps slower at 500 yards. The 7mm has 272 ft lbs less energy at the muzzle and 90 ft lbs less energy at 500 yards.

Sighted for a 200 yard zero, the WSM drops 32 inches at 500, the 06 drops 42.2 inches, the .308 drops 45.7 inches, and the 7mm-08 drops 46.9 inches.