As a long time P-38 flier, I can say with certainty that the reason why the P-38 isn't a very popular airplane in the simulator is because it is incorrectly modelled. In the simulator, given roughly equal pilots and equal energy states, the 38 doesn't outroll anything, doesn't outturn any of its popular opponents other than the P-47 and FW-190, doesn't outclimb Spitfires and Me-109s, and isn't one of the fastest ships in the planeset. To boot, it's quite fragile, it's a huge target, and locks up in a dive at any altitude. The only truly great feature of the P-38 in the simulator is the gentle stall handling (which still isn't as gentle as the real P-38's).
Don't get me wrong, it's not a miserable fighter in the simulator. It's just not good enough to keep up with the competition. It's not nearly as good as it was in real life. The real P-38 was a lot faster, turned better, did not compress below 15,000 feet, and was virtually impossible to spin unless the pilot was cross-controlling or using differential power. I recommend reading Warren Bodie's book. He's the only author to take the time to talk to Clarence "Kelly" Johnson, co-designer of the P-38. Bodie's book contains an exclusive foreword by Johnson himself, stating that it is the most accurate account he has read.
