First, you really don't need total head to size a pump. What drives the pump size is the flow rates requirement for things other than the pool itself. Examples, are spa jets and waterfalls. If the pool is the only consideration, you can go as small as you want. But if you are interested in doing the calculations, I have a few tools in the link in my sig labeled "Energy Efficient Swimming Pools".

Since you have a water feature, downsizing the pump will affect the look of the waterfall. But since you have the valve half open, I guess that is not really a problem either.

But one thing to consider is that the Superpump line is a great pump and they are all up rated and a little smaller than the label would indicate. So you have a 1.5 HP up rated pump that is similar in strength to a 3/4 HP full rated pump like a Super II or Tri-Star or Whisperflo. The Superpumps tend to be smaller pumps than a standard pump which is good news because that means it will use less energy than a typical 3/4 HP full rated pump.

If the only issue is that you have to switch the wires to go from high speed to low speed, I would keep your old pump and just get a two speed timer.

http://www.amazon.com/Intermatic-PE1...wo+speed+timer