Actually according to the PQ curves on the two pumps the only head at which there is no difference is right about 6'. From 1-5 feet the Mag5 has a substancial (10%+) flow advantage. It also gains an advantage as things get more restrictive beyond 6' as well. At 7' its only around 4% better flow but it continues to increase its margin over the Mag3 up to having twice the flow at around 9'.
Admittedly if your system basically equals a 6' head there is no difference but for more or less restrictive systems the Mag5 will increase flow.
Understand that any PQ curves supplied by the manufacturer should of course be viewed with a grain of salt
I was using the chart supplied at Marinedepot.com for the PQ curve chart. The down side is that due to using 10 more watts of power to run the Mag5 it adds more heat to the loop as well. The bottom line being.....heck if I know exactly
I do think at no point would the Mag5 actually perform worse and depending on the restriction in a loop and the type of waterblock used the Mag5 will perform better...how much better is a good question however.