No, you don't need a speed board to do 40+ knots.
In many circumstances a small slalom board is faster. For speedsurfing I use 2 boards:
-Tabou Manta 54 (54 wide, 74 litres)
-Tabou Manta 49 (49 wide, 63 litres)
I used to have a Missile XS (43 wide) but swapped for the Manta 49.
Couldn't get enough TOW in on the Missile XS due to its small size, so didn't get the time to get the trim right, before the BIG day arrives. Family, work etc.
It's better to have a board that is capable of 45 knots and then utilize 90% of that (ie 40 knots) then to have a board capable of 50 knots and only use 70% of that (35 knots).
As long as there's a little bit of chop, the Manta 54 is much easier to go fast on than the 49. Same goes for other brands. The wider surface helps to "smooth out" the chop a bit so you get to fly over it, where the narrower board tends to tilt around the longitude axis.
Just for proof, the figures of my last tuesday night "heavenly" speed session on my Manta 54
www.gps-speedsurfing.com/default.aspx?mnu=user&val=188394&uid=81Must admit this was the 2nd session on my new Gun sails GSR 6.3, and the first time I did get both the trim and my stance right.
This sails is a SERIOUS improvement over the 2013 models. I'm not sponsored, but have been using gun sails for quite a while now. This years GSR range is a leap in performance. I know sail brands (and boards too) say this every year (to boost sales) but this time it's true ;)