2 speed pumps tend to be uprated so the 1.5 HP uprated is the same as a 1 HP full rated pump. That is not to say it would not be too big for your pool. Given the pipe size that you have, I would not go much higher than 3/4 HP full rated pump. The Whisperflow is a good pump but so is the Northstar and Sta-Rite Max-E-Pro. I don't think you could go wrong with any of them.

Also, will this pump run the jets in your spa? If so, you may want to stay with the higher HP pumps even with the pipe size otherwise you will get very weak jets.