Magnums don't do well with short barrels especially with factory loads. Its tough to take advantage of all the powder with a 20" barrel and it will make your suppressor work overtime. My guess is especially with factory loads you are only going to be looking at 150ish fps increase over a 308 with a 20 inch barrel but if you every shot it suppressor it will be crazy loud with all the powder burning outside of the barrel.If you're going to put a suppressor on it anyways, why not just go 300 win mag? That should more than make the recoil manageable and then you wouldn't have to worry about energy over 600 yards. If it was my money, I'd buy a Tikka in 300, have someone cut down the barrel and thread it to whatever threads you need.