Most flew the L for its greater combat survivability. The four 50s didn't help its combat effectiveness, but more pilots lived
through engagements with the 109 F/G and 190 A/G.
That doesn't follow. You say it was better because they didn't get killed. You can't prove that, and there are so many other reasons, environmental and otherwise, that you can't make that conclussion at all.
I am reminded of the magic rock:
http://www.youtube.com/watch?v=SdBn5G7Y2RA The P-40L was not some miracle plane. They reduced the gas and lowered the ammo to 200 rpg. That's about 200lbs and change weight savings. Looking at that, 200lbs is 33 gallons. Out of a 140 gallon internal storage. So that's a little less than the difference between taking 100% and taking 75%.
We're not talking miracle performance. The actual performance specs were not much different. It was a disappointing venture because it didn't pay off. Many were built, yes. Out of need for airframes rather than because it was better than what it replaced.
EDIT: The P-40E had 148gal, but looking it up now the P-40F that the L was based on was more like 157gallons. That means the impact on fuel weight was less significant than before.