Shooting at longer distances basically means being a rocket scientist - we're talking Mach 3+.
So ballistic coefficient becomes important. The flat-butt bullets I like to shoot at 200 yards ... maybe I got lucky.
The best bullet I've found at 6.5 is the Hornady I think I put a link to - 123 grain. BC = .510
At 308, you have to go up to about 180 grain to get a comparable BC.
However, there's another factor besides BC. Sometimes the slightly wider opening of the bigger brass allows its geometry to perform better.
You get zero reward for using the smaller diameter caliber.
The 270 WSM maxes out at about 3000 ft pounds.
The 300 WSM maxes out at 3500 foot pounds.
Top speed for both ? 3250 fps.
http://www.midsouthshooterssupply.co...sion-20-Rounds
http://www.midsouthshooterssupply.co...oint-20-Rounds
There is something about the slightly larger opening that helps get a lot better performance, from the same internal volume of the brass.
I don't know if I have the budget to work through all this.
I also don't know if I'll ever shoot at a distance (greater than 200 yards) where BC really matters.
Fun to talk about though - helps me understand guns better !