Low speed pump is not only good for saving electricity bill. Low speed means a better filtering, and probably you could have a clearer water. The effectiveness of filtering depends on the flow-speed of water (and filter area). If you have too much speed it is like passing the car inside a car wash at 20 miles per hour. Even if you pass many times the car will be badly washed.
About VSP vs dual-speed, I dont find many advantages of Variable over Dual. I think probably dual-speed is enough for 90% of people: one low speed (tipical 1500 rpm 250-350W) for 80% of time and one high speed (tipical 3000 rpm (1 - 2 KW) for clear surface and maintenance tasks. But please. its only an opinion. I would like to hear different reasoning.
You can´t run VSP at speeds much less than 1.000 rpm in a pool. Very low speeds even can affect refrigeration of the motor.
About electrical consumtion for same flow rate, everyone is right. It has more to be with "efficiency" of pump (how many electricity watts for same phisical work), than size. Usually bigger pumps has better efficience (but not allways, depend on models, quality components, etc.). And there are performance curves depending on hydraulic conditions of the circuit... I.e. same pump can waste more or less depending on the dirt on the filter.
I am very surpised by the low cost of VSP you post. if you see comments in the page it is not "energy star", and they put another model (more cost), that probably has better "efficiency".
VSP are really a "Frequency Variator" and a basic "three-phase-motor fixed-speed pump" (you could even build one VSP just by buying this two components separately). So, taking into account the cost of these two components, low cost of pump you post make me be suspect about the quality of the pump component.
