So they managed to retain good performance by using a dual-staged supercharger with twin-speeds?
As I recall, the 2-stage 60 series Merlin was introduced in the Mk 9 Spitfire. This was necessary in 1941 to counter the threat of the new FW-190 which completely outperformed the Mk-5 Spit with its single-stage Merlin.
It should be noted that the first 2-stage supercharger was on the P & W R-1830 in the F4F-3 Wildcat. Unlike the AAC, the US Navy didn't pursue turbochargers or V12 engines ("we'll buy liquid-cooled aircraft engines when we convert to air-cooled submarine engines"). Ironically, the Navy found their lower altitude pacific war didn't require the two-stage blowers & later FM Wildcats (built by GM) used single stage Wright 1820's. They were lighter, cheaper, simpler & developed more sea level power to get the FM Wildcats off the short decks of escort carriers.