If your gun shoots as good as you say at 300, I think you would be fine even with your 7mm (I'm assuming it is a magnum) for most realistic shots while deer hunting. What stubblejumper said is exactly right. IMO, you'd have to be an incredible shot, with tons of practice to consistently shoot good at 800yds. Not many folks could actually do it. Heck even shooting 400-500 yds is tough, even with good equipment and plenty of practice. Nothing personal, but to say you want to shoot 800 yds at deer is probably about as stupid of statement as I've ever heard. Even at 300-400 it is very easy to miss or wound one, even if you are an accomplished shooter. Sounds more like a case of ego to me. But....if you are absolutely sold on having to have the biggest and baddest out there, get the sendero in .300 ultra mag. Then with alot of practice, and an extremely sore shoulder, and an empty wallet, you may be able to take some game at long range.
A better option IMO, practice with the one you have, possibly have it accurized, and most of all....learn how to get the deer in closer, you'll be alot better off.