Yes, the twin boom configuration was naturally the most practical solution, my argument was more or less academic. (I do think the airfoil chosen for the P-38 was it's main development limitation -and not just due to compressibility, but that's not the nature of this discussion)
The coment that the turbochargers had to be used because they "
had a long engine." ??? That makes no sense to me at all.
The turbochargers had to be used to obtain good high altitude performance. (the V-1710 being equipped with a relatively small supercharger, though compared to the early Merlins and DB-601 it's critical altitude wasn't much lower, all being rated for "medium altitudes")
Everyone but the US was using single-stage supercharging on their engines, up until the Merlin 60 series came out. Though single stage two-speed (though never Used on the Allison) or variable speed (DB engines) were faily commonly used.
The only country using 2-stage superchargers operationally in the early stages of the war was in fact the US, with the good old F4F's P&W R-1830-76/86 featuring a 2-stage 3-speed (including neutral) supercharger with intercooler. (some models, particulalr for foreign use, featured engines -Wright 1820 or P&W 1830- with simpler single stage 2-speed units, and the FM-2 with it's Wright engine featured one such as well -resulting in slightly poorer altitude performance)
The Corsair's R-2800 featured a similar 2-stage supercharger.(along with a more powerful one in the F4U-4)
Had Allison focused on such developments, thy too probably could have had a decent 2-stage supercharger available fairly early on. (though USAAC policy focused on turbocharging -technically all the engine companyies had to do was make sure the engine would be suitable for a turbocharger installation, the R-2600 being an example of an engine with poor turbocharging qualities)
Then of course came the 2-stage Merlins.
But no operational German engines ever featured 2-stage superchargers or intercoolers (inless the rare use of turbocharging). However, due to the larger displacement of the DB-605 (at similar size and weight to the Merlin) it was still able to compete for performance at high altitude. (particularly with the larger DB-603 supercharger of the AS and D models, and of course, GM-1 provided further high altitude booting possibilities for German engines)
And turbocharging in pretty much any case on a WWII aircraft was actually 2-stage, with the turbocharger (after passing through the intercooler) feeding into an "integral" "engine supercharger."
The best altitude performance the single stage V-1710 managed was with the supercharger gear ratio increaded from 8.8:1 to 9.6:1 on engines like the the V-1710-81/99/85/83, which increased WEP to 1,480 hp at 10,500 ft. (mil 1,125 at 17,500 ft). But due to increased charge heating max boost was limited to 57," the older engines (-39, 35, 73 etc) had a WEP limit of 60" with ~1,570 hp, but only at ~5,000 ft with mil of 1,150 hp at ~14,000 ft. (all figures with ram air -high speed level flight, w/out ram -low-peed/climb figures are all 2,000 ft lower)
This performance was almost as good as Merlin 45 (single-stage single speed as well), but max power is still a bit short as well as critical altitude. But not bad with all things considered.
Also note that with the 8.8:1 blower, the actual boost (MAP) limit was above 70" Hg, but this results in almost 1,800 hp, well above the 1,600 hp rating for the reduction gearing (not to mention other structural areas) and in any case this would only be atainable at SL with full ram. (or over-rev to 3,200 rpm)
http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/Allison 1710-39 abuse.pdf