Not even a comparison the 800 BD will walk all over the 850 at elevation. Your looking at a 30-40 hp different at elevation depending on boost level. BD should be close to 200 plus on low boost.
I’ve had 6 or 7 Axys now 4 with turbos that have a combined over 12000 miles the highest being 6000 and the only warranty claim was a Tstat on one. 850 Axys lasted 3 days lol. Hopefully 2020 is fixed but in my experience so far the turbo 800 BD would be a much more reliable sled and way less down time then the 850. I’m giving the 850 one last chance this year but after last year I’m not holding my breath for reliability.
I have a ‘17 800 Axys w/Silber Turbo running 5# spring and wouldn’t trade it for stock 850 ever ! Love it.
However would like to get new 2020 850 and boost it someday! Boost has a way of ruining a guy when he rides NA sleds