If I'm windsurfing on smaller gear in high winds, I probably mean that there's so much power in the sail that I can't manage it. I lack the skill to stop the universe from exploding when I'm sailing a 4.0 in 35 knots. Better men (and the Moreno twins) may be able to hold it down, but I can not.
If I'm on mid-sized or larger gear (anything from the promiscuous 5.5 on up) I don't say I'm overpowered until the sail's aerodynamics start to break down. You can feel and hear the thing cease to be a wing providing lift as it becomes a tortured and abused shape in a gale. A sudden increase in speed and power that, knot for knot, would scare me off of a given high-wind sail is, on bigger gear...exciting!
If gusts hitting the sail makes the rig feel like a punching bag, I'll opt for lots more downhaul to let the sail spill the bursts of power. If the challenge is keeping the board on the water, I'll swap in a smaller fin (or even a smaller board.) But if I'm afraid to bear off into a jibe or try a jump, it's usually time to rig down.
(I can't remember the last time I was overpowered. We're in a wind deee-rought on Long Island!)