I can agree with the observation that speeding only saves you insignificant amounts of time if your total commute time or absolute speed are rather low. But once you're driving anywhere for an hour or more and at speeds above, say, 60mph or so, it appears to be more up to interpretation about what "significant" means.

I wouldn't be surprised at all if people who are already inclined to speed were encouraged to do even more by your app, though I suspect there'd be a sampling bias in that those people might not download it in the first place.