If 15 GPM is the maximum demand for the heat pump, then a 20 GPM pump would be perfect for the heat pump/house use. A standard 3450 RPM pump will last much longer than the 10,600 RPM SQE. All you have to do is limit the cycles per day to make a standard pump last a long time. A big pressure tank will probably not limit the cycling enough by itself. Three or four big tanks would. However, big tank(s) can cause problems with the flow through the heat pump. With a big tank the pressure will continue to vary between 40 and 60 PSI. As the pressure varies, the controlled flow rate through the heat pump will also vary. The house would also see this variation in pressure. And the variation in pressure means the pump is still cycling.
A small tank and a CSV can be used with a 20 GPM pump. The CSV will maintain constant pressure for the heat pump and the house. The tank will supply water for small intermittent uses of water like rinsing a toothbrush or filling the ice maker. When long term uses of water are required, as for the heat pump or a shower, the CSV will maintain a constant pressure and keep the pump from cycling.
The SQE has only been available for about 11 years. I doubt that you will find anyone with an 11 year old SQE. Even if there are a couple that old, the SQE has had no less than 5 updates or major change ups since it began. They donít make changes to them continually because they are working properly but, because they continue to have problems. Some areas they seem to last longer than others but, I think you will find that 3 years is about the average life expectancy. A standard 3450 RPM pump with a CSV to limit the cycling could last 20+ years. Figure the cost of replacing the pump every 3-5 years compared to a pump system that will last 20 years, and you will see why longevity of the pump equipment is the most important thing required for a heat pump to be saving energy.