My old man has a 1970s Yamaha 2 stroke 15hp.
It is light as a feather compared to my de - restricted Tohatsu 15hp 2010 4 stroke.
I regretted getting a 15hp almost right away as my Sib struggled to plane with any weight onboard.
From the outset I knew that the option to change the carb was always there if the 15hp was not enough. There's not a massive gulf in power output bwtween 15hp and 20hp but if your going to put your back out - you might as well do it lifting a 20hp.