I can't why see why it would make any difference, but I would say it would depend on the airplane. If its something in the likes of a piston single, then yeah I can agree that 20 knots could make life interesting.
On the other hand, if it's a twin, I would say that you could stand another 10 knots or so.
In general, I'd venture to say that a good general answer would be 30 knots or higher is "strong". Depending on your geographic location, your thoughts may vary. 30 knots in mountainous terrain is different than 30 knots over nebraska.