Enough of the transatlantic willy-waving already!*
As I understood it the Griffon and Merlin designs were nothing more nor less than the result of RR having two different design teams working on separate concepts in the late '30s, with the Air Ministry simply trying to make the best of the resources at its disposal at any given time.
From
Wikipedia:
Unlike the Merlin, the Griffon was designed from the outset to use a single-stage supercharger driven by a two-speed, hydraulically operated gearbox; the production versions, the Griffon II, III, IV and VI series, were designed to give their maximum power at low altitudes and were mainly used by the Fleet Air Arm. The Griffon 60, 70 and 80 series featured two-stage supercharging and achieved their maximum power at low to medium altitudes.
[* - Because, let's face it, advances in engine development from about 1936 onwards in the UK were driven by necessity, even if few would mention it aloud.]