Expected top speed with 420 hp mercruiser?

[Riboffsore]

The new Mercruiser 420 hp is a interesting engine. And me and my friends wondering what topspeed you can expect from one together with a 8-9 m hull (stepped) and a weight of around 1000 kg without engine?

The boat is a ribboat Not build yet!!! a new one...

 

[Scand]

The Goldfish 29 claims 66 knots with the same engine, but the top speed obviously depends the configuration of the steps, deadrise angle etc.
The aerodynamics on a standup rib like this will not be brilliant so there will be something to be gained by going to a hard deck design.
If top speed is the main objective then a surface drive is also worth considering.