One thing you will notice at distance is the 300 WM will be as accurate or better against the .308. The heavier the bullet the longer it takes to settle down. By settle down the bullet does not come out of the muzzle clean but has a yaw to it.
With 155's my 300 WM is pretty accurate at 100-200 yards. It will group <1 moa all day. When using 190 MK's it takes close to 300 yards for the bullet to settle down and give me better than 1 moa accuracy.
The gun BTW is a Blaser LRS2.
Just so you realize the 300 Win Mag is brutal on your shoulder if you spend any time shooting it. The .308 is a lot more comfortable to do any extended shooting with. There is no animal in the commonwealth that a .308 can't drop with one shot.
I will tell you there are very few guns out there that will give you the out of the box accuracy that the PSS will for the price.