I cannot say I understand the math. It is a function of the pump and pipe size which I think should be 3"-4" that deep (hence the long vertical 'tank'). Check out this website:
500ft+ - Premium Pumps USA | premiumpupsusa.com . There are dozens, probably hundreds, of pump suppliers out there that sell 500' plus pumps that go WAY past 1.5gpm.
Questions I would ask:
Why the limited gpm? Unless you already know it is simply a weak well. If so, the rest of this is moot.
What is the pressure and flow at the head?
What horsepower/gpm pump are they using?
What is the pipe size and what is the distance from the pump to the house?
Is the bladder tank working properly?
Assuming you have adequate electrical service (a 3 horsepower pump at 220VAC single phase is going to pull around 17-18 amps) this should not be an issue either.
Relocation the well is just not an option? Is sounds like ya'll are up on a pretty big hill.