Allied strategic bombing was usually done from high altitude, regardless. The B-17 and B-24 had greater range at higher altitudes.
Unfortunately, Axis fighter aircraft such as the M-109 and FW-110 were able to reach the same altitudes. For that matter, the German 88mm anti-aircraft gun could put a cloud of flak up that high in the sky.
In the aircraft of that era, service ceiling was largely limited by the engine air intake. Put a supercharger on the engine and you can climb higher. American fighter aircraft were late to the supercharger game: We didn't have a good high-altitude fighter until the British put their magnificent Rolls-Royce Merlin engine into North American Aviation's P-51 Mustang. The result was magic...