On low speed you won't have 60 ft of head. The dealer probably meant that with 60 feet of head at high speed at low speed it would deliver 50 GPM. Pump affinity laws tell us that:

The head on low speed will be 1/4 of the head at high speed

The GPM at low speed will be 1/2 of the GPM at high speed

So if you have 60 feet of head currently with a 2HP uprated pump, then according to the pump head curve shown here, the Whisperflow should perform as follows:

High speed head = 60 ft.
High speed flow = 100 GPM
Low speed head = 15 ft.
Low speed flow = 50 GPM

The flow rate of the pipes are recommendations not actual limits. Although very inefficient, you can go way above those values without too much problem. However, the pipes will wear out faster but it should happen over a fairly long period of time.