Whats surprising, according to that graph, the 151/20 isnt much better than the FF or the tater. Its a bit weird couse they "feel" totally different in game.
What may distort the perception of the graph by a little is that there are two very different weapon sets in it: One high velocity group (MK 103, 0.50 cal, BK-5) and the rest. If I would take out the HV group and would show only the 'ordinary' guns, the differences between the MG 151/20 and the MG-FF would seem to be somewhat bigger.
But then, according to the data from
Erprobungstelle Rechlin, both rounds are't exactly worlds apart.
MG-FFM, 2cm Minen m Zerleger: 695 m/sec muzzle velocity, time to 400m 0.8 seconds
MG 151/20, 2cm Minen m Zerleger: 775 m/sec muzzle velocity, time to 400m 0.7 seconds
(The Mk 103 Minen projectile would take just over 0.5 seconds for the same distance)