More expansion often means less penetration, because of the increased frontal area. For example, out of 8 bullets shot at 50 and 400yds from a 300wm only one had more penetration at 50yds then it did at 400 (and the only one that penetrated more at 50 had a smaller expanded diameter!)
http://www.rifleshootermag.com/ammo/ballistics-test-best-300-win-mag-loads-market/
Considering the 400yd impacts are in the 2300-2400fps range, which is comparable to 308 speeds at normal hunting ranges, i feel like an argument can definitely be made that a 308 will out penetrate a 300wm with the same bullet.
(I'm not arguing a 308 will kill better, with lower energy and less expansion, just pointing out that more velocity doesn't equate to more penetration with fast moving, expanding bullets)