Tyhe allison got a 2-stage supercharger (albeit only a primative one at first, w/out intercooler and other such advancements as the Merlin-61) by the end of the war V-1710's had advanced superchargers available, though some of these (particularly those used on post-war P-82s) were more probematic than the Merlin.
The simple 2-stage supercharger (auxillery supercharger) sucessfully raised the critical altitude to ~23,000-25,000 ft (~21,000 ft for WEP) and such models were used in all production P-63A/C's the C's engine was particularly powerful with 1,500 HP WEP and 1,800 HP with water injection at SL.
It was the USAAC's policy of relying on turbo chargers (as they saw the advantage over conventional superchargers) for high altitude performance and had Allison focus on low-altitude superchargers for all else. Thus the V-1710 go a late start on development of other designs.
The V-1710 also used an integral single-speed supercharger, ulike the 2-speed supercharger of all but very early (Mk.I-III iirc) single-stage Merlins. The single stage supercharger could be tuned for higher critical altitudes with the disadvantage of restricted throttle setings down low (to prevent overboost). For example the V-1710-81 that powered the P-40M/N and P-51A, actually gave higher performance at medium altitudes and similar performance at 20,000 ft as the Merlin engined P-40s. It was limited to 1,200 hp for takeoff but at 5,000 ft it could produce its full WEP of 1,480 hp which could be maintained up to 10,400 ft. Millitary power of 1,125 hp could be produced up to 17,500 ft, and the engine was still making 836 hp at 25,100 ft.
With this engine the P-51A could actually outpeform the early P-51B of the same time period (give or take a couple months) at medium altitudes. With 415 mph at 10,400 ft in WEP, and 408 mph at 17,500 ft in Mil power. Range was about the same with the same fuel load. With 67" boost the P-51B was about equal at these altitudes, granted the A was lighter with a lighter engine, and no intercooler and acompanied ducting but the V-1710 had ~10-15" less boost at critical WEP altitude. Even at 25,100 ft the P-51A was making a respectable 395 mph with 836 hp!
See:
Mustang (Allison Engine) Performance Trials http://www.wwiiaircraftperformance.org/mustang/p-51a-1-6007.jpg http://www.wwiiaircraftperformance.org/mustang/p-51b-engdiv-na-flighttestdata.jpg