With a 6.5 and my Hyper 111/34cm drake I have done 34.3 knots. I belive they can be very fast in the right conditions.
Its interesting to see guys doing 32 knots with an 8.5, gives me something to aim for! I think the real limitations are simply where you sail. Where i go it gets choppy, sand banks come along to quick when you bear off, and the wind is extremely gusty.