I don't know that what you expect is reasonable if you're on MTB trails and similar, running or biking.
The best GPS watch I've ever had was the 910XT, closely followed by the 310XT in terms of track accuracy. They were consistently within half a lane (that is, not only on the right side of the road but also on the correct margin of said road) for running when it came to track accuracy, and distance was bang-on every single time. It also did great on hikes in the backcountry, with the biggest limiting factor being battery life.
Right up until I ran an Xterra race, in the woods, on a double-black-diamond MTB trail.
At which point the distance was off by about a quarter-mile for a 5k (short; I was over the finish at well short of the 3 mile chime!) AND the track was nastily-off on later examination too. The race organizers warned of this in the pre-race briefing -- that one should not trust pace, distance or anything else if wearing a GPS watch, as they were going to be disastrously wrong.
That was a MEASURED 5k; the watch is just flat wrong, and if you look at the track you can see it's a mess -- yes, the course is full of switchbacks, one after another, it just plain gets it wrong in fairly serious fashion, including marking you 30' or more from where you really are.
I had a LOT of activity on that 910XT and its track and distance performance were simply outstanding -- materially better than anything else wrist-worn that I've ever used. On an out-and-back when running on a sidewalk or runner's lane it would frequently put the two plots literally on top of one another even with the zoom level set to 5m! In other words fixes were accurate on a 90% confidence level to within a couple of feet, not meters. You can distinguish on these tracks that I was running on the sidewalk as opposed to the road if in fact I was -- every time.
Yet that very same device, in an Xterra 5k, produced a horrifyingly bad track and distance error that was so egregiously out-of-bounds as to be completely unusable for anything including pacing (since pace was obviously off too.)
I think you're asking the impossible, in short, where low power consumption and wrist-based size is part of the design requirement. A bike-mounted GPS can have much more power available to it, thus a larger and more powerful (and quieter) pre-amp, a better positioned (and larger) antenna, and more. A wrist-worn device cannot meet any of that, ever, irrespective of the design from my experience, in that even the best of the best that I've ever seen simply could not perform as you seek under those conditions.
