If you want one to play with, I'd never consider anything higher than a 4x lens for 3D. The lens I use when competing open class on my Axcel sight is a 3x.

One big thing to consider................

A lens may/may not change impact point. If it rains, and your sighted in w/ a lens........it gets messy quickly, and if you pull it out it may/may not change your impact point. Also, what are you going to do when you pull down on a 20 yard turkey.......and all you see is black? This is a huge majority of the shots you are going to see from HC stakes anyhow........things at less than 30 yards.