If you're talking about both fighters extending after the merge and making long-term climbs, the Spitfire will win since it is faster and climbs better. If you're talking about both fighters making an Immelmann after the merge, the Spitfire will win because it Immelmanns faster and tighter. If you're talking about the P-38 going straight up after the merge and attempting to rope the Spitfire, then the Spitfire will still win because, although he may stall before the P-38 does, he will have plenty of time to hose the P-38 before either of them stalls.
In short, I don't have a large problem fighting in the vertical. I'm not as good as Murdr or Ack-Ack, but I can beat quite a few of the other respected fliers in this community as often as they beat me, in a fair fight - using the vertical. And I beat more Spitfires than Spitfires beat me. But if you're regularly beating Spitfire Mark XVIs in an equal energy dogfight, you're not flying against pilots who know their ship.