A couple more notes on the engines. There was a similar development to the R-2000 earlier on, the R-2180 "Twin Hornet" basicly the 14-cylinder predicessor to the R-2800, however it proved troublesome, largely in power output falling short of expectations. (iirc ~1,300 hp rather than ovver 1,400 hp) It was abandoned in favor of developing the R-2800. (there was also a later, successful R-2180, but based on the R-4360 rather than the original design)
Again the R-2000 hadn't originally been planned as a "beefed up" 1830, rather a low compression version capable of similar power (slightly more) with lower octane fuel. As things turned out they ended up using it with higher octane fuel at higher boost preassures, with later versions tuned for this. (up to ~1,600 hp WEP iirc)
I wouldn't call the R-2600 that advanced, AFIK it never really developed into anything other than an average, slightly lower power, mediaum altitude (though this was largely a supercharger limitation) alternative to the R-2800. Of course, it's an older design, but it's also a bit wider than the 2800, though la bit lighter too. (and about the same diameter as the R-1820 Cyclone, which was, of course, much wider than the 1830)
A decent engine design, maybe even good for some fighter disigns early war, but not particularly impressive, OK as a bedium altitude bomber/attack plane engine, or for transports. (and perfromance was fairly comperable to the single-stage supercharged medium/low alt R-2800's)
As to the P-39's limitations and the Allison, I agree somewhat, but a big problem with the V-1710 was lack of a good mechanical supercharger available. Sure the USAAC wanted to focus on turbos, but Wright and P&W were smart enough to put development into superchargers. (P&W in particular, wich had the first 2-stage unit in service I beleive, with the Wildcat's R-1830)
I think mechanical supercharger were generally better suited for fighters, even in cases like the P-38 and P-47 (which had particularly few issues) they would have had a good many benefits frm having high performance 2-stage superchargers instead. (reduction in weight for one, the P-47 would loose all that ducting, and "gut" reducing drag and range should be improved significantly) Especially if they'd focused on supercharger high out-put versions sooner, even if they'd started at a similar point as the early Corsair;'s 2-stage 2800, boost limitations, water-injection, and supercharger improvements would likely have come sooner if that version had been focused on.
I t would have hurt performance above ~25,000 ft (especially with the F4U-1's engine), but reduced weight and drag would have other improvements, low alt performance improves, and most combat took place below this height anyway. (though engagement may have started around there) There should be no problems cruising higher for providing top cover either.
Again on the P-39 though the USAAC also made the CoG problems worse (leading to the lower stability and stall characteristics) by lengthening the fuselage. Cutting the wingspan increased wingloading and reduced cruise efficiency to some extent (lower aspect ratio= higher induced drag), and the lower canopy reduced visibility and made it hard for taller pilots (above 5'6" iirc) to fit in the plane comfortably. The only positive change the AAC made was the increased fin area (vertical stabilizer), solving the lateral instability. (though this would have been a natural modification anyway)
Moving the radiator intakes to the wing roots probably didn't hurt ither. (though it may have elimiated possible added fuel storage in that location)