But most of the time that ATC ask us to "keep the speed up" below F100 we are already inside controlled airspace and it has to be said that this happens fairly frequently. However, I think the original question implies VFR flight outside of controlled airspace.
I was once persuaded by my boss to come back from Beauvais VFR (to save a 3-hour slot delay) and we did most of it at 240 knots. I swore that I would never do it again. It was not smart.
As to the previous altercation, I cannot help thinking about being asked to do 280 knots in a PBY-5A! That would probably only be possible if the entire crew were wearing high visibility jackets.