Willem,
to my knowledge the record stands at 19.6 knots or was it 19.1 knots. This is an average speed from Weymouth Speed Week in the UK held by Detelf Mohr and Bodo v.Schrader.
The distance was 500 m if I remeber correctly.
I myself have asked that same question doing some trial but never exceeded 19.5 knots top speed. Also on cats there must be some hull length related top speed like on monos.
My TIGER as an example goes a little faster than the 16 but still not really beyond 20 knots top speed.
Fritz-R. Klocke