I love the Nordhavn's as well, I ran a 40 for the owner ,took out guests on day cruises
I understand you concerns on speed
They sure are low on maintenance though ,and cheap to run
Give Selene's a look ,they usually have a bit more brightwork to deal with,but have higher hp engines to give a bit more speed
The 57 that I ran would do 10kn @ 50% engine power (Qsm11), so would be capable of 13
What size range are you considering?