Lots of good advice already posted about pros/cons of diff sized HP outboards, so no need to rehash anything. I guess all that I can add that is hopefully relevant to the discussion is had you ever considered a lower speed/higher torque pitch for the prop? You said one of the key selling points to getting the 15HP was the extra power for getting the mother-ship out of trouble if needed, but at the same time the power can be intimidating when running flat-out. I've never tried straight up towing a hull your size with just a 15 HP, but you can for sure push/pull the bow or stern to help in tight maneuvering in dicey conditions. Short shaft outboards are factory equipped with a prop pitch for the 'average' customer, which typically means flat out best speed (even though a factory rep will say they strike a balance between efficiency, power, and speed... no, no... its just speed, trust me).
So you could benefit from a pitch that is geared more toward torque which gives you the most bang for you buck out of that 15HP when you are pushing the mother-ship hull around... Also, you mentioned runs for provisions and water jugs. When you increase torque you will sacrifice top speed, by how much depends on how far you tip the scales in the direction of torque. This will also effect your fuel efficiency. With less speed you won't get up on plane as fast, more drag=greater fuel consumption (thus the term 'hole shot')... however, with more weight in the boat the extra torque improves your hole shot, but you still will have less top speed; its all quid pro quo.
You also mentioned the weight of the motor is problematic. On your size sailboat do you not have a set of davits that can handle the weight of the motor first, then lift and stow the RIB itself? I imagine you can lift it, you just can't swing it and stow it easily from there, so I guess I answered my own question.
