no. mag drive or direct drive isnt going to matter, if your talking gallons per hour vs head height, just go by how the pump is rated. the only thing is that with the pump your talking about, that extra flow could cause a problem so i would suggest putting a valve just after the pump so you have some control, but other than that, dont buy what he's sell'n cause its a bunch of BS!
Its pretty obvious that whoever told you that is a salesman, and not an engineer.
oh, and just as something that you can throw back at them, some of our submersable pumps can be run submersable or IN LINE and either way its still a magnetically driven impellor, and either way its ratings are still pretty valid as far as flow rates go(theres a tiny bit of loss using it inline vs submerged)
further more, if it were true that you have more losses per foot of head height on mag drive vs direct driven centrifugal pumps, then that would mean that the magneticly driven impellor would have to be slipping and i can assure you that this is not the case.
That would be called magnetic cogging, just like with a stepper motor. It would be a destructive and noisy occourence, and you would know when it happened.
Basically what would be happening, if this were to occour, would be that the impellor has a magnetic north, and south(or multiples of each) and inside the motor casing is a winding of wire, or multiple windings, these are called the fields, or coils. These are energized when you plug the pump in.
What happens is that the magnetic flux of north and south, that is generated by the coils or fields "rotate" around the impellor centerline due to the winding direction, # of windings, and the fact that you have plugged it into AC power.
This magnetic rotation, or commutation, pulls the magnetic rotor around in a circle around its central axis, same as the flux generated by the coil(s). The 2 are coupled, and rotate in a 1 to 1 ratio.
IF the impellor were to "SLIP" or cog, this would mean that the impellor would fall behind in its rotation enough degrees such that the impellor would basically slip or stall and fall behind untill the rotating magnetif flux catches up to it and tries to spin it again. This action is accompanied with a violent and sudden stop of the impellor, and thus is also accompanied by a noise.
Tell him hes full of it... you dont have to drill your sump, unless you just wanna... just use the bigger mag pump, put a valve on it for control, and smile, cause now you know more about how it works than he does.