Please excuse me, I know I'm not a ribber
When I was looking for insurance for my formula I spoke to a few co's at the boat show. Basically (And I am quoting here!) they think we all exagerrate about our boat's speed and think it's faster than it is.
However, if you know the boat's weight, can guess the hull form reasonably, and HP, it's pretty easy to make a good stab at the speed.
If it weighs 800kg (350kg hull, 100kg engine, 2 blokes @100kg each, 150kg for batteries, fuel, anchor and other kit) with 75hp flat out speed should 35 knots, maybe slightly more. It 'may' do 40 if you lose a good bit of weight from my sums and the hull is better than I've assumed, you tweak it for top speed, etc etc.
Often however the insurance co's say 'design speed' rather than top speed. So in that case whatever the manfr's tell you is safe.