[Kurfürst]

Regarding 1, the German engines were of typically high compression ratio engines (starting at 6.7:1 and increasing 8.5:1 during the war) against the typical Merlin/Allison 6:1.

Regarding 2, lack of oxygene that can no longer be provided by the supercharger as it runs out of air (get it?) above its rated altitude is one of the reasons for the drop of power with altitude. The other cause is the supercharger itself - unless we talk of single stage superchargers, the second high altitude speed/stage of the supercharger always takes away more power from the propeller to drive the supercharger harder.

There is some inherent handycap with the radial engines themselves, as typically radial engined fighter require more power to develop the same performance as inline ones, it also means the engine needs to develop more power - and thus it also needs more air to burn more fuel, and thus having greater need for a high capacity supercharger than an inline engine.