Depends, size, load, how lifted it is, conditions ect. but Im guessing around 40.
Based on where my 2 toon 20' pontoon tops out with a 90hp. I can get high 20s/low 30...extra 60hp and lifted same size boat with just me should be 30% faster.
Might try asking over at thehulltruth.com - they should be able to give ideas.