The standard 5th generation 60GB iPod battery is 600mah, and so your 850mah battery should be better than stock.
That being said, I can get 10 hours with my stock battery and the conditions you describe (assuming minimal to no backlight usage.)
It is fair to note, though, there are two major variables in your equation. The use of a highly experimental, unofficial, unsupported build being variable #1. (Though early reports show no battery issues with the COP patch.) A no-name battery which claims to be 850mah being variable #2.