I've done some stall fighting in a Spit 9 against a P-38. With some manuevers, the P-38 has the edge (for example looping like that)... with some others the Spitfire has the edge.
Easiest way for the Spitfire to win is to go vertical, period. The Spit 9 can climb, and be steady, at 70-80 mph. It is tough to get behind a plane doing 70-80 mph. Every time the P-38 looked like he was going to get a shot on me, I'd break into him and climb, and start scissoring (like a flat scissors, except I kept climbing steeper than he could) if he tried to follow me. If he didn't, I'd just continue my turn and come down behind him.
The P-38 actually comes pretty close to being as good as a Spit 9 in a stall fight, but it isn't as good by any means. Plus one other gigantic disadvantage is the P-38s size, it is considerably easier to hit a P-38 with a snapshot than a Spitfire.