GB is a semi-displacement hull and will plane with enough horsepower. The theoretical top-speed is infinite.
The difference is fuel burn. If it took 45 litres to get somewhere in 3 hours or 45 litres to get to the same place in one hour (sadly, its not linear) its the same thing...
The best GB42 I have seen had a single 6 cal Gardner that idled at 400 rpm and came with a drum of Brasso.
Don't believe everything that you think.