I have a 3-15x Premier on my .260 and have shot all the way out to a mile with it. I agree that if shooting tiny groups is your main focus more magnification is probably better, but for practical field style shooting more than 20x is probably a waste.
I had a Bushnell 3.5-21 DMR for a while and liked the higher top end but not enough to replace the Premier on my .260.
For a field rifle I think the most important part of the scope is having a reticle you know well and having an adjustment system that requires as little thought as possible (ie single turn knobs, matching reticle/knobs).