I'm assuming the quoted stats are for a twin tip, so do we add roughly 5 knots to the low end for a surfboard?

I guess that also depends on what size surfboard you are using....

I think a surfboard reduces your kite size by 2. So if you ride in a particular wind range on a TT with an 11m kite, then on a surfboard you could probably use a 9m of the same type kite. This is quite a rough estimate as it depends on board size.