Most of the stuff we have played with have been in the 18-21 range.
Most recently we have been doing a few 50' boats with twin 480s running 21x32 Acme props for speeds in the 64-67 mph range. Granted we have been running ASD8's so we are not limited in diameter.
the 39 nortech that the boss has is running 18X32's I believe and that boat runs in the 80-83 range consistently.
The only Nortechs running the bravos and diesels are the 43's with triple 6lp 315s. those are limited on diameter as well.
All I am getting at is the majority of stuff we run, perform better when you can take advantage of the larger diameter.