35-50mm stem depending on whether you feel the bike needs to be shorter or not. to be honest unless your well over 6ft I dont feel there is any need for 800 or even 780mm bars. 740-750 is a good size for most people.
I've been loving a 35mm stem and some 780mm wide bars lately. Even managed to take 5 seconds off my best time for my favorite downhill section. Managed to reduce my record from 3:35 to a 3:30 on my 3rd attempt. I've done over 100 timed runs of this section and always considered anything faster than a 3:40 to be absolutely ripping.
Also, I've always considered myself to be a half size to big for my bike(6'2" on a large frame) so i was super surprises to find that i didn't feel cramped or to far back.
Bar width vs shoulder width and also bar backsweep have a large impact on bike fit. 10mm of extra bar width can have a huge effect on reach, in my opinion, about the same effect as 5mm of stem length. Of course this effect would be different between people as we all have different width shoulders.
I'm 6'3 with a34" inseam on a L Tracer 275C with a 24.25" TTube. I went from the stock 70mm stem and 740 bar to 50mm and 800. The bike feels so good with this set up. Stable, nimble, and I can keep my lower back in a flat, pain free position now. The DH characteristics of the bike now feel perfect to me. Even better, the flatter xc portions of my ride are much more fun. I lose a little uphill on the very steepest of uphill climbs. I'm talking about super steep granny gear pitches. My typical rides are 1.5 hrs up/ level and 30 down.
I'm completely sold on the short stem/wide bar concept. The enduro type bikes really shine with this setup. I like it so much I'm going to experiment with my pure xc Trek Fuel 29er. I'll try a 70 stem on some 750 ish bars and see if that makes the DH a bit sportier without hurting the uphill beast that is my Fuel.