Colin,
As you ask about the 767:
There is no limitation on its use in terms of speed. Part of the drill for an overspeed condition is t deploy speed brakes first before retarding the thrust levers. This does two things, one increase drag and two cause the nose to pitch up as the centre of pressure (the point through which lift acts) moves. Nose goes up, reducing speed rapidly.
Company SOP is no speedbrake use beyond Flap 20.
On the 757s we have fitted with winglets there is a limitation on full speed brake deployment - 330 Knots Indicated Airspeed. Maximum of 50% deployment only is permitted beyond this speed. There is an auto stow facility which limits the spoiler movement above this speed. This limit is to do with the bending of the wing caused by the winglets combined with the loss of lift at the root of the wing at such high speeds. Winglets place a greater bending force on the outer portion of the wing upward than on aircraft without therefore when you deploy the spoilers at the centre of the wing you effectively increase this bending force placing extra stress on the structure. This is not desirable at such speeds.
The speedbrakes on the 757 are not that effective at low speeds with the aircraft being so overpowered. At lower speeds 220kts or so it is little more than a rumble maker.