Whether you go with a 20,15, or even a 10 hp outboard, buy it with a flatter pitched prop than standard and consider adding a Doel Fin.
Some years ago with a 9.9 hp Toahatsu 2 cycle on a 9.5' RIB I couldn't get up on plane with more than one person aboard. I changed the prop to a flatter pitched one and that helped. A Doel Fin helped even more, about equal to the flatter pitch. After both changes I could easily plane with 350 lbs of people, a 40 lb dog, 30 lbs of groceries and 50 lbs of gas.
With today's 4 cycle engines which inherently have less low speed torque, it is even more important to use a flatter pitch prop which lets the engine rev up into its higher torque zone to get up on plane easier.