According to Bill Gunston, it took five years to develop functional engines and get them into production. That means your engine goes into development in 1936. Wilth the 2020 vision of hindsight, the people who enlarged otherwise conventional engines (P&W R2800 and RR Griffon) did better than the people who did exotic cylinder layouts like the RR Vulture, Napier Sabre, and the DB 606 and 610.
Then again, it took five to six years to make the Sabre reliable, which is about you would expect, sleeve valves or no sleeve valves. The Merlin its share of troubles, but they started early enough that they were sorted out by the time the war started.