It all depends on you.
I can hit a 5 gallon bucket at 500 yards with my Aimpoint equipped Colt 6920.
Of course, it takes a couple of shots to walk it in to get the holdover/wind compensation right.
All magnification does it make the target look X times closer. It won't help you shoot better, but it will help you see better.
If the optic has hash marks in it, it will make remembering holdovers easier though.
Less, "Hmmm, that looks about right, BANG, oops, little higher, BANG oops too high."