Expecting 40knots is probably about right with 150hp on a 6.6mtr. Just be aware you're in the zone where you start throwing big power increases at it and gaining relatively little top end.
I can get 38kn on the 6.5mtr with 130hp. but far more important for me is being able to cruise at 25kn at around 3800RPM. I'm not going to spend much time flat out and the difference between being able to do 38 knots and 40 knots is kind of irrelevant.
The minimum requirement for planning is abount 50hp/ton so if you aim for twice that you shouldn't have any issues.