Even if a map is installed I find a TCX offers more fidelity- GPX in the 810 means for some courses the Garmin does not follow roads, which is not useful
That's more likely to be due to the creator of the track, no? Mine will occasionally beep off course at me because whoever point the points in did a rough old job. But I don't recall my 800 not following roads. I'd still be lost in the Chilterns somewhere if that was the case.
Even if a map is installed I find a TCX offers more fidelity- GPX in the 810 means for some courses the Garmin does not follow roads, which is not useful