15 is plenty, (if not maybe even a tad overweight?) I used a 15hp on a 3.6m plastic pontoon as its main motor. I wanted to test its limits with two people on board at went out rock hopping in 22knt wind. Overall weight was probably in the 350-400kg range with all the water and two soaked spearo's on board. Got what I was asking for and puttered home burning 1l per km. Got home nonetheless, but upgraded since to 30hp.

So yes - you will get a RiB home with 15hp and then some. Question is if you want to carry it with you in the first place… 35kg extra is quite a number