Where to start....
Acceleration, OIO, I suggest you test it. My tests showed the P-38 winning at all speeds, slow and fast. The margin was small, but the excess speed potential and thrust of the P-38 won at all altitudes I tested (sea-level, 10K and 18K). As I said, the margin was small though, not enough to escape with (though no acceleration margin in AH is enough, not even the vaunted La7). Spit does tend to be in the top 10 accelerators but that's a pretty crowded group with very close times (measured in portions of a second).
No arguement about Spit E retention, but you can't compare any plane in the game to the Spit in that regards. nothing compares (the Tempest is close for a while). 90% of the time when I think a Spit out-accelerated me in a P-38 it's actually because he came out of the initial merge with more speed because of his superior E retention.
Firepower on the P-38 is totally fine. 1 hispano vs 2, true, but also twice the .50's with twice the ammo for each gun. Center mounting also does make a difference, to say otherwise is silly. If you want to compare it as inferior, why not throw in something with 4 hispanos. I think the 2 vs 2 contest at the convention was won on a D1.1 shot with a P-38.... I know I've actually made my longest kill with a P-38 at ranges over D1.0
I don't know that I've ever spun a P-38 because of flaps. I tend to use them in very short bursts, only seconds long, so my speed doesn't have time to vary much though. I have had them retract though in the middle of a shot (especially around 250mph). Thing is, I know that at 260mph I don't need flaps anymore, they simply don't help. They don't improve turn-rate since you are G limited at 260mph anyway, they don't improve turn-radius (same reason), etc.... below 250mph they work (and do improve turn, radius) but they won't retract until technically I don't need them anymore. I rarely use more than 1 notch of flap, usually only at the top of a zoom or in an absolute emergency since the drag penalty is somewhat extreme.
Comparing to a Spit is somewhat unfair anyway. He has way more turn-potential at all speeds than the P-38, sustained. He's capable of pulling something around 34 deg/sec at 220mph while your BEST will be more in the 30 deg/sec at 240mph (with flaps). that's decisive in the Spits favor.
Hammerhead, the F4U was probably riding the flaps pretty hard too... he has the same issues as you do and his snapstall is way worse. I simply haven't seen the snapstall
-Soda
Aces High Trainer Corps
The Assassins.