Generally is a rather broad term. Excluding proper fuel management is not really a good basis for the comparison either.
The USAAF data almost always shows the Mustang being tested at less than full combat weight, especially the later models with more guns, armor, and fuel. Empty the 85 gallon fuselage tank behind the cockpit and you have a different plane. Around 600# of weight in a bad place for the center of gravity changes the whole flight envelope.
The first P-38J, the J-1-Lo had 65 gallon leading edge tanks, the later versions had from 110 up to 160 gallon leading edge tanks.
As to flying for fuel economy, the speed of the P-38 was not greatly reduced when flying at 50" of pressure at 2200 RPM in auto lean, and it could cruise at the same speed as the P-51. Using the Lindberg method was out of the question in the ETO, since he specified auto lean, 55" of manifold pressure, and 1600 RPM. The only problem with using the first method is that you had to be able to throttle up without looking, and to know when to do it. If you had to look at the throttles, prop and mixture controls, and had to be told when to throttle up, you were in trouble. The P-51 was easier to fly in that respect. But there was no great disparity in cruise speeds, and if you had to fly with the bombers, then the bombers were the limit anyway.
Also loaded out for the same mission, the P-38 could climb out towards enemy territory directly, where as the P-51 required you either reach enemy territory with lower altitude, or burn fuel circling while you climbed.
To quote Captain Arthur Heiden 20th FG 8th USAAF, "You could reach cruise altitude from England before landfall in a P-38, while in the P-51, you still had considerable climbing left to do." I'd prefer to arrive to the fight at altitude and speed, with plenty of internal fuel left to fight and go home.
At its critical altitude of of 26,900 feet, the P-51D had a top speed of 437MPH, while the P-38J-25-Lo had a top speed of 414 MPH. At 30,000 feet, the speed roles were reversed, with the P-38 at 412, and the P-51 at 404. At 35,000 feet, they were no closer, at 389 for the P-38, and 378 for the P-51. At 40,000 feet the P-38 was at 348 MPH, while the P-51 was at... just over 37,000 feet. These compare true air speeds at WEP, 67" of manifold pressure for the P-51, and 64" for the P-38. The P-51 had neither the lift nor the power of the P-38 at very high altitudes, but the better prop carried it up okay.
The advantage of the P-38 was two fold. the turbocharger gave it a decided power advantage at altitudes of 30,000 feet or higher, and the high aspect ratio wing gave it more lift at low speed and at high altitude in the thin air.
The Merlin was always a great engine in the altitude range it was tuned for. The single drawback of a strictly mechanical supercharger compared to a turbocharger is that it is tied directly to engine speed, and is incapable of compensating for changes in altitude beyond a very specific range. that is why there were several versions of the Spitfire that had only engine changes, their engines were tuned for certain altitude ranges.
The thinner laminar flow wing of the P-51 gave it a decided edge in speed and drag, but it had less lift, which hurt lower speed handling and also hurt the climb rate.
Nothing is free. the high aspect ratio wing of the P-38 gave it lift, fuel capacity, low speed handling, and climb, but at the price of drag, and it was the wing that caused compression. Kelly Johnson always admired the Spitfire wing profile, but the Spitfire never had the range afforded by the internal wing tanks.
The first thing done to the P-38 to combat compression was to increase the radius at the joint between the wings and the center nacelle. They had wanted to further increase that radius, along with moving the leading edge at the center nacelle forward, thereby increasing the sweep, and add two or three more degrees of dihedral to the wing section. But any really serious development geared towards major air frame changes was dropped after the P-38K was cancelled.
You are correct, the P-38 had no exhaust thrust boost, nor did it have any ram effect, except for a field modification never factory nor Lockheed approved, but used none the less. The GE B2,B33, and B34 turbochargers did not like the backpressure resulting form the fitting of an exhaust hood designed to produce exhaust thrust. There was no gain from the radiator exhaust either.
You are also correct that the Curtiss prop on the P-38 sucked. It is indeed a testament to the Allison that it was able to power the plane so well despite the props. It should be noted that Lockheed requested that they be allowed to fit the Hamilton Standard prop to the P-38. The USAAF and War Production Board considered the P-38 to be so critical to the war effort that the estimated production stoppage of 7-10 days was out of the question. The USAAF supplied the Curtiss prop in 1939, and required Lockheed to use it, and while they allowed nearly every other fighter to be fitted with the Hamilton Standard prop, the P-38 was never afforded that advantage.
A good comparison can be made by looking at the P-51K, which was fitted with a version of the Curtiss prop. The P-51K lagged some 25-40 MPH behind the P-51D in top speed, and some 600 FPM in rate of climb.
Witness the P-38K, AKA the P-38 that never was. With just the Hamilton Standard prop, it was faster at all altitudes than the P-51D. It also had a rate of climb superior to any U.S. fighter. The increase in efficiency from the prop resulted in a 10-15% increase in range.
I'm in complete agreement that exceeding critical Mach in a P-38 was more dangerous than in any other plane save the P-47, and it was a close race there. of the U.S. planes, the P-51 probably did as well or better than any when critical Mach was exceeded.
I also agree that between 20,000 feet and 27,000 feet, at speeds above 300 MPH, the P-51 held some decided advantages in both turn and roll rate, along with a measurable advantage in engine performance relating to top speed. The advantages the P-38 held were climb rate, turn rate below 275 MPH, and acceleration below 350 MPH.
Below 20,000 feet, and above 30,000 feet the P-38 was a better ride.
That being said, knowing half a dozen real World War II pilots who flew the P-51, the P-38, and the P-47, from what they've told me, I'd prefer to be in the P-38, as it was a better all around performer at most speeds and altitudes. I'll give up a little advantage at certain speeds and altitudes in order to gain better all around performance at a wider range of speeds and altitudes.
The truth is, it's the guy with the yoke or stick in his hands that makes the difference. Among the U.S. planes, the P-38 was like a Ferrari or a Corvette, it was a killer in the hands of a dedicated expert, but would get an uninitiated kid in deep trouble.
There is considerable data on the P/F-82. The engines in it were actually designed for the P-38, and fitted with turbochargers would have made considerably more power than even the P-38L engines, but the Curtiss prop could never harness the power.
