Your HP rating for the P-38L is the commonly published military power rating, and not the actual War Emergency Power rating. The WEP rating for the -30 Allison in the P-38L is 1725HP. The P-38J engines had a verified 1612HP at WEP, and that was verified in testing by the USAAF, not just claimed by Lockheed and Allison. It is extremely doubtful the later L model would have nearly 300HP per engine less than the earlier J model.
According to Warren Bodie, Lockheed engineer and author of "The Lockheed P-38 Lightning", a book considered to be the final authority on the P-38, the P-38L had -30 Allison engines rated at 1725HP in WEP, and a top speed of around 440MPH at 20K-23K feet, according to Lockheed and Allison data. Bodie and the Lockheed documents are THE source for P-38 data, Warren Bodie was a Lockheed engineer for decades, worked in the Lockheed Skunk Works, and knew many of the engineers and pilots directly involved in the P-38 from beginning to end, personally.
And 1725HP per engine is 1725HP per engine, it does not matter how many there are. The power loading on the P-38 was better than on the P-51. The P-38L had 4.92HP/LB, the P-51D had 5.30HP/LB. The P-38 had a high aspect ratio wing that the P-51 did not. The P-38 would easily outclimb the P-51, and up to about 350MPH, it would out accelerate it.
The only problem with the "cold weather" theory about European operations is that at 25K feet, it is just as cold over the equator as it is over Europe. It's also just as cold over the south west Pacific ocean area at 25K feet as it is at 25K feet over Europe. The P-38 operated successfully, even in the dead of winter, in Alaska.
The reason the 8th AF found the Allison temperamental was they failed to operate it properly. Lockheed test pilot Tony Levier found that not only were pilots flying the P-38 at the wrong settings, but the maintenance crews were not properly adjusting the turbochargers or the fuel system.
The correct settings for cruise were 1600RPM, 32" MAP, and autolean. Pilots were flying the P-38 in cruise at 2500RPM, 15" MAP, and autorich. It was using more than twice the amount of fuel, fouling spark plugs, flooding fuel into the engine oil, keeping the coolant too cold, and keeping the oil too cool. Pilots went into combat with an engine that for the most part was not even warmed up to proper operating temperature. When they slammed the throttles open, they had practically no boost, the spark plugs were fouled, and the engines were dead cold.
Further, the crews had incorrectly set the turbocharger adjustments, so that the engines bucked and surged almost all the time. Not only that, but the fuel was poorly mixed, and the tetraeathyl lead was not homogeneous in the gasoline, so the octane could be as much as 20 points low.
After Levier demonstrated the correct settings and showed the crews the correct way to test and adjust the systems, pilots who had been complaining of rough engines and had come home with blown engines on a tea cup full of fuel now came home bragging how well the engines ran, and with 100 or more gallons of fuel to spare.
Also, not all of the P-38's in Europe had engines replaced, that is a complete exagerration. For example, Art Heiden flew his plane "Lucky Lady' (seen on postage stamps) for 324 hours of combat missions, without a single abort, and without a single engine change. Richard Loenhert flew his plane "California Cutie" for hundreds of hours with no aborts and no engine changes as well. It was not nearly so uncommon as you might think.
