Being able to see bullet holes in paper at 600 yards is asking a lot of a spotter, in my opinion. Conditions will be a big factor, however. Humidity and heat = mirage.
I doubt you will find a $500 spotter that do what you are asking, even with ideal conditions. I have a Swaro HD 65mm 20-60 power spotter that I can't pick out bullet holes in paper at 600. But where I live the air is heavy and humid with too much mirage. I shoot steel at that distance and the impacts are easily seen.
Yeah the conditions must be different, I'm in canada.If you can see your bullet holes at 400 with your 22X scope, you may be able to see them at 600 with a spotter. I can't overstate how atmospheric conditions will affect your optics. You must be shooting in better conditions than where I live in Louisiana.
Also, those 'Shoot N See' type of targets are much easier to see vs plain white paper.