ok, i understood all of that until you got to the merlin engine part. it was designed for down low fights, ok got that down(heh no pun intended). but what about a griffon engine? werent those supposed to help up higher or is my WW2 engine history off? as for the clipped wings, i didnt know that they actually helped with the roll rates. that really got me intrested, thanks for posting. 
It depends on the blower attached to the engine. The Merlin 61 in the Spitfire Mk IX has a critical altitude of about 27,000ft, about the same as the Griffon 65 in the Spitfire Mk XIV or the Merlin 71 and 72 in the Mosquito Mk XVI. The Merlin 70 used by the Spitfire HF.Mk IX and HF.Mk VIII had an even higher critical altitude. The Merlin 66 in the Spitfire Mk VIII and XVI has a critical altitude of about 18,000ft. The Merlin 25s in the Mosquito Mk VI have a critical altitude of about 13,000ft. If I recall correctly, the Merlins in the Spitfire Mk VIII, Spitfire Mk IX, Spitfire Mk XVI, P-51B, P-51D and Mosquito Mk XVI have two speed, two stage superchargers for better performance at their critical altitudes. The Merlins in the Spitfire Mk I, Spitfire Mk V, Seafire Mk II, Hurricane Mk I, Hurricane Mk IIc, Hurricane Mk IId and Mosquito Mk VI have a single speed, two stage supercharger. I am not sure which group the Lancaster or P-40F falls into, but I believe it is the second group.