It doesn't have to be flat water for speed to be a safety factor. Watching Ryan Finn sail the 36' proa Jzerro north off the coast of Chile after rounding Cape Horn, frequently averaging between 10 to 12 knots, it was obvious that speed was an advantage as he was battered by storm after storm with "waves in excess of 20 feet". It sure helps though, in those conditions, to have a bigger boat.Maybe the question should be, "how fast would I really like to be going in the current sea state?" If its flat water, yes please, I'll go as fast as possible without flipping. If it's sporty and I'm with some hardy souls who enjoy that kind of thing then let the games begin!
I remember sailing the 76 foot catamaran Double Bullet II from Kauai to Marina Del Rey (1995) when we hit a 24 hour stretch of 18 foot seas. As I recall, boat speed was in the high teens which allowed us to steer comfortably around breaking waves, choosing a safe path rather than wallowing and being hit by them.