"Modern sailboard"???
There is a huge range in modern sailboard types. The guys you are watching may be on wave or free style boards, they may not be trying to go upwind, or they simply may not be very good. A modern Formula board (planing course racing board) will be competetive with or beat most cats around a race course (upwind and downwind).
The only time I've seen kites go upwind better than windsurfers is when the kiter is powered up and the windsurfer isn't (the opposite would be true if the windsurfer were powered up and the kiter not).
Around here, most of the kiters I see like to do "downwinders".
sm