What if BMW decided to take a similar route in their radial engine development that the Pratt & Whitney company did with the R-2800 'Double Wasp'? That is they decide to develop an 18 cylinder 46 liter displacement engine that is pretty much based on bolting two Wasp engines together. The BMW 801 engine was a 14 cylinder engine with two rows of 7 cylinders with a 41.5 liter displacement, which is basically the same idea as the R-2800, but without the 'full Wasp'.
https://en.wikipedia.org/wiki/Pratt_&_Whitney_R-2800_Double_Wasp
https://en.wikipedia.org/wiki/BMW_802
https://en.wikipedia.org/wiki/BMW_801
So if BMW takes this route they would end up with an engine (assuming a similar development pattern as the BMW 801 and R-2800) that is ready in 1940 for serial production, but not widespread service introduction; by late 1941 the engine is made reliable and ready to go for full service.
Assuming it has a similar output as the R-2800, the BMW 802 would deliver 2000hp with the basic 87 octane B4 fuel that the LW used early in the war (and the US used before it got 'richer' mixtures later on). It could fit into the Fw190 frame too:
http://www.luft46.com/fw/fwbmw802.html
Later developments, including the introduction of the higher octane C3 fuel (IIRC it was 96 octane and gave between 10-15% more horsepower with the same engine), would boost power output, not to mention the introduction of MW-50 boost (similar to the US WEP setting and gave 500hp extra boost for 10 minutes with a 5 minute rest between uses, max 2 uses per flight).
Assuming the Luft '46 entry is right, then this version of the Fw190 (1944 AFAIK) would have a top speed of 450mph and would be able to maintain its power up to 39,000 feet with a three speed, single stage supercharger (with later developments boosting this performance). Having something this powerful in 1942-43 would be a serious boon to the LW at low and high altitudes, as a 2000hp engine in the early versions would put it way ahead of anything the Soviets or Western Allies were fielding in 1942. By 1944 when the P51D shows up the standard FW190 would be able to match or exceed its performance, with less attrition in previous years meaning that more experienced pilots would be around.
It also means much better bomber performance not to mention night fighters (which could catch up to the Mosquito with +2000hp engines for aircraft like the He219 or Ju88G).
Was this a viable path?