Re: 250kts on departure
It's standard SOP in the US, at least at every major airport I've ever operated into or out of. Never been given "no speed restriction" below 10,000' when arriving or departing from an airport over there.
With reference to the 'heavy' jets requiring greater than 250kts min clean speed, I've never had any ATC restrict us to the 250kts on departure. On the B744 at typical T/O weights before an Atlantic crossing, our min clean speed will be around 260-275 knots. We always climb at 250 or min clean, whichever is greater. Occasionally, ATC may want to know what our min clean speed will be for their own decision making reasons but invariably, they are well aware that a heavy a/c will be slightly above 250kt min clean.
As has been pointed out already, the typical time difference of reaching a boundary or entry/exit point between 250kts and 300kts is around one minute or less depending how quickly you can reach 10,000'.
Thoughts on the restriction? Won't really make much difference. What is preferable is more direct routings.