The turbos were removed from the P-39 do to cooling problems, and the fact that the plane was not going to be used at High altitude. Also the USAAC could get more hours per engine before it cratered. The same reason that they De-rated the later 1710 Allisons. Even today in the modern warbirds that are using the 1710's most owners opt to have them derated so that they don't have to pay 500,000 dollars for a new one as often.
An Allison V-1710 doesn't cost $500K. I've seen them selling for $200K or so.
The G series Allison V-1710 in the P-82 Twin Mustang is rated at a higher HP than any of the F series Allison V-1710 engines found the rest of the World War II aircraft. Two differences made a little difference. They added center counterweights to the crankshaft (F series engines got those crankshafts at times, late in the war) so that the engine was smoother and easier on bearings at higher RPM. The intake manifold was altered several times beginning with the F-17 series from the P-38J, to improve mixture distribution which prevents detonation.
The turbocharger was removed from the P-39 because the USAAC decided that the scoop and the cover for the turbocharger caused too much aerodynamic drag.
The problem with turning up the boost on a V-1710 Allison wasn't really the engine itself, especially after the updated intake manifolds. Usually it was a problem with fuel quality combined with maintenance procedures. Properly tuned an Allison V-1710 can easily run as much as 80" MAP in stock form, on decent fuel.
