???

Does the 810 have any issues with the things that the 510 (or the 500, even) does?

As far as I know, they all work equally as well for those things.

That is, you already have a 510/500 in the 810 (though, slightly bigger).

The maps might be useful, even without turn-by-turn working.

Downloading Openstreetmap maps is easy but bicycle paths still might be spotty (depending on where you live) and they can make navigation more problematic.

(I bought my 800 for maps/navigation and it works very well for me. I've tried a Touring and that appears to work as well. The problems Touring has seems to be mostly due to problems with the Openstreetmap maps it uses.)