A couple of questions.
1. Why are you trying to take game at up to 800yds? Are you positive you can make a clean kill at that range?
2. It doesn't matter how much you spend on a high end scope if you expect it to make any shot count. Do you already have long range experience in calculating wind drift, ambient temp. and a sundry of other variables?
Here's a ballistics chart that shows a 32 ft. drop at 800yds. using factory ammo. You will have to be able to hold over that much and most scopes don't have that much adj. available. http://gundata.org/ballistic-calculator/
Good luck to you an I hope you figure it all out.