It depends on how large the variance is, which I suppose you could measure first. You could even show the existing distribution (or typical range) within the progress bar.

A somewhat related approach is used by build servers showing a progress bar of the current build base on how long the previous build(s) of the same project took.

That would work quite well for low-volatility tasks. iOS upgrades is a pretty good example, for one. In PC-land it gets a bit more difficult due to variations in hardware; see video games.

In my opinion, it's not worth the telemetry.