I'd say 0.5-2 depending on the shooter and how much further out from 800m it is. As I noted, at 1000-1100m I'm fairly confident that my shot will land in the upper body of the target when I'm wet, tired and uncomfortable(4 days of 3 degrees celsius in the day, melting snow etc, and -5 degrees celsius and the wet clothes freezing on you isn't fun when you're waiting to take your shot), while some would miss all the time, most likely. There are some who are really pushing the envelope and extend the range where they can be fairly confident to hit in the chest to 1100-1200m, and can definitely take chances to hit targets beyond 1200m, and still have a decent chance of hitting, my instructor one of them.
One of the big problems is the precision fit of the sabots. That's one of the areas where there are only a few rare humans that can compete with machines. Hand-loaders will have a problem with getting the perfect fit necessary for defeating the problem. There's also another problem I've heard being talked about, and that's some people having a low ratio of twist for the caliber. At one swedish forum there was talk about sabot ammo, and for example with 7.62x51, those who have 1 in 12 twist complained more about sabot ammo being inaccurate than those with 1 in 10 twist. Probably because the slower twist doesn't yield enough force to make a quick and clean separation.