If you have been given speed 250kts or less, then I would imagine that a delay would have also been passed so why would you want to accelerate to 300kts+ before coming back on the speed? There is a debate about whether ATC can give aircraft an indicated speed to transition at rather than allowing the aircraft to automatically convert at whatever altitude it desires, however there are obvious arguments against (brick walls). I don't mind aircraft accelerating beyond the speed I have issued, I just hope that the extended mileage it then accrues from a couple of rather harsh turns isn't minded either.