While the EFI system on the 800 should start the sled regardless of temp and elevation, it's possible the temps are just too hot to compensate for the heat. With that kind of temp, it's likely the air/fuel mix is too rich to start. Have you tried holding the throttle open when starting? I would also look at plugs, it could be that unless your plugs are perfect, there's not an adequate spark to fire the rich air/fuel mix... Just a SWAG, mind you...