They were right about the walther P38. It's still arguably regarded as one of the best all around handguns of the 20th century. In the 9mm regime, it wins hands down. It wasn't until the 1990's that companies finally started topping it. I'd rate the browning high power (though still second to the p38) and colt 1911 in there too.
As for the 20mm and 30mm of the German fighters... Don't confuse muzzle velocity with range. Can't say I know what the range of the 20mm and 30mm was, but they were lesser muzzle velocities which means they'd have to be lobbing them in. Of course, if the Germans were to shoot from a high AoA, that wouldn't matter much. I would agree that the 20mm and the 30mm's main advantage layed in increase snapshot damage.
MiniD