I think there a number of other factors that I would consider:

Crew Ability
Temperature (Air and Water)
Equipment (Dry suit vs. Wet Suit, etc.)
Available Help

I think the most important thing to consider is available help. I feel a lot more comfortable when a few catamarans are together and are accountable for each other.

That being said, the maximum I would consider in ideal conditions would be about 25 mph. On a cooler day with my young daughter on board and little help available, I might not go out in 12 mph.

-Tim