I think that the answer is part boat, part confidence, and part crew.
I agree with Tim, the boat will take more than all of you can take.
Personally, I am confident alone in 6 footers to 8 footers (Lake Michigan) and 35 kts, but i prefer no more than 5 footers and 25 kts.
With experienced friends or my son 10 foot and high 30's. With my wife 3 foot and 15 knots....my wife will hurt me more than the boat!!! Learn to understand the boat, your crew, and yourself. Smile when it hurts so the crew does not know you are nervous. Have fun!