Your Garmin help, please

The trouble is, it depends so much on how you use them. If I keep on proddling my 800, turning the backlight on and switching pages all the time, I can kill the battery in under 8 hours. Leave to do its own sweet thing, and just look at one page of numbers, and it'll do considerably more. So doing a proper back-to-back empirical test would be tricky.

Anyway, if the battery life's not enough then just buy another one and switch over at a convenient moment...

There is no convenient moment when you're racing.
Extended battery packs are a better option. I'll stick with the 800s.