Can't help on the sailing...
qualities of the P33, but I heard it is a fairly decent boat and when raced in it's PHRF class, can do well, FWIW.
As fara as the Horses for the displacement, I think 15 hp may be a little on the light side. My Pearson 10M has a 27 hp and it displaces about 13,000 lbs. I have heard that the general rule of thumb is 2 hp for every 1000 lb of displacement.
15 HP will get you in and out of the harbor on a light/moderate day, but if you have to push through a lot of current, heavy chop, or strong winds, you may not have the HP to do.
A few years back, I went up a a river with the tide coming out. The river has a natural current of about 3 kts. At max ebb the current can get as high as 6 kts. It must have been close to that when I went up it as I had my motor at full power and was only doing 1 kt (SOG) with a two bladed prop. My hull speed is 7.2. Had I had a smaller motor, I doubt I could have done that.
If you liked my advice/response, give me some rep points. If you didn't......well what do you expect, it was free.