I have a remark regarding the pure theoretical approach, without considering the efficiency of the foil, the vibrations, ect ...
Peter is right in a sense that the hydrofoil principle has no limited speed in itself ; so we could believe that by infinitelly reducing the thrust, your leg power and the cavitation, or whatever you want, we could reach infinite speeds ; same for the kite, it is not theoretically limited in terms of pure speed if its structure allows enormous loading and if the wind is high enough.
BUT ! there is a physical limit actually. It is linked to the Lift/Drag ratio of the kite. If you do the calcs and complete projection of forces, you will see that the rider speed will never overpass the multiplication [L/D ratio * real wind speed]. So here is a definite top speed limit, that will never ever be reached whatever improvement you do on the foil. For illustrating example, a standard race kite (assuming L/D=10) in 30 knots will allows up to 300 knots speed
so we have still "some margin" with our current speeds
, as of course physics like A/R of the foil, drags in water and air, vibration, legs muscles, ect ... will limit this speed far sooner .... but can still be improved step by step if you progress in all fields.