A heat pump on a 15' pool? The heater will cost more than the pool! If you have the room, a 4'x10' solar panel can be plumbed in pretty easily and should warm your pool more than adequately.

Smaller pump/ bigger filter: Happy pool owner. Bigger pump/smaller filter: Problems. Horsepower is GREAT in cars, but not so good in pools, especially smaller ones. Your pool is just over 5k gallons. My 16x40 is about 19k gallons and I run a 1 HP pump (OK, it's an IG pump, not an AG, so it's more like a 1.5 AG pump). But I run it at low speed almost constantly, 12 hrs/day and that's like 1/4 to 1/3 hp pump.

Carl