I think the M15 is fine for three or even more on a casual day-sail outing if you aren't concerned about comfort or performance. You're only a hundred pounds shy of doubling the weight of the boat (a good assortment of snacks and beverages might get you there), and the M15 will perform accordingly. Stand the heavy guy in the hatchway, as you suggest. Place yourself and the other person according to conditions (light=opposite each other, heavy=both to windward, but as far forward a possible in either situation). You'll take more water up through the cb drain regardless. Wear your flip flops...it'll go away. I've never seen the question posed or answered about what is the optimum capacity for either the M15 or M17, but maybe I've missed it. t