I've spent some time exploring the outer limits of 308 Win peformance.
1500 yards will always be a low percentage first-round hit probability on a 1-moa size target, for a 308. I don't believe there is any getting around that. At that distance it's running out of gas even with the best LR loads. Any changes in temp, mv breeze etc is going to make a significant change in POI. I only shoot the 308 that far for the challenge and amusement.
I shoot a moly'd 208 AMax seated to 3.08" oal, over 49.0 gr RL17, in Win brass. This gives 2620 fps from my 22" Rem varmint barrel.
This load is surprisingly accurate and consistent to past 1500 yards, at 4500' elev. Same load shot well to 1500 meters at 2900' elevation.
Probably the best bullet I've played with in the 308 for really going long has been the 215gr Berger. I've only played with a handful of them, but made 2600 fps with a 26" barrel and RL17. G1 BC is in the .690's, anyone can run the calcs on that and see it will be running 1200+ fps out to 1500 yards even at sea-level.
I only pushed to 215 Berger to 1200 yards, at 4500' el, and it was very accurate at that distance, shooting at sub-moa size rocks. It's a bullet I need to play with some more one of these days.
Here's some more info on the 215gr Berger I had posted a while back when I shot the few I had.
http://www.longrangehunting.com/forums/f19/215gr-berger-hybrid-308win-104197/