The fan was necessary for cooling, at the end enabling it to have tighter cowling?
The power loss at high gear full throttle height and above was what, some 50 HP? The Jumo 213A did not have any special supercharger system, but a simple 1-stage, 2-speed*. Out of the 3 major things that determine engine's power (displacement, rpm and manifold pressure), the 213 had 2 (displacement, rpm). Not that it didn't have the MAP, it did, but the other 2 were engine's high points; the 2-stage variant were real beasts above 8 km.
I can agree the 801 have had an, say, unfortunate layout of the intakes - the air was entering intakes after being deisturbed by many parts of the engine itself. The full throttle height (at 645 km/h) was some 600 m higher vs. a static engine, not a major achievement (801D); so with ram effect it's providing those 1440 PS at 6300 m. The D-9 was gaining almost 1500 m, even if it's using the Steig Kampflesistung (speed 675 km/h); engine providing 1540 PS at 6600 m (with ram). Fairly comparable with P-51B/D gains through usage of ram effect.
The A-9's BMW-801S was providing 1700 PS at 6400 m (with ram), gain being modest 700 m. The 801 really needed those intakes from the Fw-190A-3/U-7...
Thanks for reminding me to that important 'ingredient' of the ww2 planes' performance. A small addition by yours truly: the faster the plane, the better harvesting of the ram effect. So the lower drag played part here too.
The line for the 801D in my post above is after the fan power is deduced; I'm not sure for the 801S. (help!)
*same with the 801C/D/S, among others