Great post Cumbrian, and a point I was trying to get across, but perhaps not very well.
A bullet that fails to stop in it's target is only dumping a fraction of the energy it's carrying. The majority of the bullets energy is dumped into the backstop.
I'd guess, assuming a similar bullet construction(ie, a traditional softpoint), there is only a finite amount of energy a carcass will absorb, before the bullet exits. I'd be very, very surprised if the energy absorbed is that much greater when hit with a 338 over a 308 - assuming it exits which in both cases they almost certainly will. Infact, I'd bet it's possible that a smaller calibre, or lighter bullet that is more likely to stop, could impart more energy on the target, than a big heavy round that zip's on through.....
However, change bullet construction, to say a ballstic tip, and it'd be entirely different.