I just can't see how they would work well enough in a non-Pro Racing atmosphere to justify the cost.
In a typical qualifying session, we'll sit on grid for over 5 minutes prior to going out; and if you want to be at the front of the line, you better be there 15 minutes ahead.
For a race, you need to at least be on grid by the 5 min mark, usually because 5 min is not really 5 WHOLE minutes if every one is in place. They'll make it 3 minutes just to get ahead on the schedule.
The waiting time alone pretty much negates any advantage of having the warmers.