Two kiters have had peak speed of over 60knots and several at over 58knots:http://www.gps-kitesurfing.com/default
. ... peedType=9
So it is possible, however, the right conditions to sustain this for 500m are hard to find.
Yes more efficiency is required.
Kite speed sailors are by far the most efficient on a knot per $ ranking though:) which to me is way more impressive than a knot per wind strength ratio
I had a quick tot up of the price of windsurf speed gear.
NP 5.4m EvoV sail £850
Carbon Boom £650
100% carbon 4m mast £525
Mistral speed board about £1200
Mast base and ext. about £100
Fin about £150
Total about £3500
Genetrix 7m Hydra with bar 1150 Euro (£930)
Only production speedboard I could find was the 2010 Underground at $700 without straps. (£440) Most top guys use customs so probably more expensive. At a guess £700?
Pads straps and fins about £150
Total about £1750?
So roughly half the price of windsurf speed.