BUG_EAF322, Kaz,
The Spit's wing loading is much, much less than that of the P-38. Long wings mean jack squat in and of themselves. Long wings typically mean lower wing loading that shorter wings, but that comparison is only reliable if you stick to the same airframe.
BTW, I recommend that you read up on Spits. The F.Mk IX, HF.MK IX and Mk XIV all have very good high altitude performance, your P-38 doesn't rule the roost alone. The Spit IX in AH is the F.Mk IX, not the common LF.Mk IX.