The reason for the 45 GPM rating for 1.5" pipe is the velocity of the water. It is preferred to have your water moving at 8' per second or less. In 1.5" pipe 45 GPM of water moves just under 8' per second. At faster speeds the friction loss per foot of pipe increases at a faster rate, so pumps operate more efficiently under 8' per second. And also, the friction loss is so great that normally the head will increase by enough that the flow will be reduced to approximately 45 gpm.

The Whisperflo is a better pump the Hayward Super pump. It also pumps more water when comparing HP to HP. The 3/4 HP Whisperflo pumps 70 GPM @ 50' of head. The 1 HP Super pump pushes 50 GPM @ 50' of head. The Whisperflo is quieter on high speed and I've always found it to be a more heavy-duty pump. The Whisperflo is meant to compete with the Hayward Northstar and the Pentair Super Flo is meant to compete with a Super Pump.

Brad