Quote from Mr. Neil Pryde in this article: www.rsxclass.org/mr-neil-pryde-windsurfing-as-an-olympic-sport/
"5/ The RSX Class is without doubt the most athletically demanding event and the equipment is the most technical and high performance of any current Olympic Class, racing in wind speeds of 3 to 30 knots and capable of speeds exceeding 35 knots. The competitions are very close and demand a high level of tactical skills."
I find it hard to believe that a RS-X would ever go over 35 knots in an Olympic course type of race. As I understand it, even Formula boards rarely get over 30 knots.
I dont know personally, having never riden one, but I think that perhaps it might on a flat speed course with the appropriate rig, fin and heavy rider.
Can anyone who has sailed these things with a GPS confirm this or enlighten us? 
There is a video on YouTube where they did speedruns in a harbour.
Think Dorian won the challenge with his Olympic rig
Max 32kts
Hans Kreisel has done 33.67kts Vmax (2sec.) on it: www.gps-speedsurfing.com/default.aspx?mnu=user&val=171035&uid=2042
I have never tried one though..
The new RS:Convertible is a modern slalom board with a foil option, I have seen one in action (foil mode) and the rider said to me that its a quite nice slalom board also..
I guess they mean an RSX is technically able to reach 35k. I doubt if they ever reach those speeds during races, there will be some one out there who can make an RSX do 35k in the right conditions on the right day.
Probably not fair to compare Formula with RSX as they are different board design, but dedicated formula guys have been hitting mid 30k's for a while.
Average Joe (me) has been just shy off 33k peaks on my old F161/9.5m and it felt faster and more scary than 40k odd on a 50 litre speedboard, nearly ** myself, but in a good way!!!
www.gps-speedsurfing.com/default.aspx?mnu=rankings&Year=-&Tab=0&Gender=0&WeightClass=0&AgeClass=0&SpeedType=9
