5000hp / 12 cylinders = 416.666 hp per cyl.... Wow!

Considering that for an equal rpm the 2-stroker has twice as many power strokes most likely has something to do with it. Horsepower = torque x rpm. I'd like to find out what the max RPM of the Crecy was...

I haven't heard of the 2-cycle vs 4-cycle displacement problem, but it does state the 5000 hp horsepower rating at the RR engine website Peashooter posted:

Crecy – V12, 90°, sleeve-valve, liquid cooled, 26.1 litre, supercharged, two-stroke, mechanical fuel injection (from Focke Wolf 190) Otto-engine. The 1942 prototype had surprisingly low fuel consumption at the highest power output. In December of 1945, after eight years of development, work on the Crecy was terminated. At that time the Crecy was capable of double the horsepower of conventional 4-stroke designs of the same displacement. Only six Crecy engines were built, all with even serial numbers (2, 4, 6, 8, 10 and 12). This has led to confusion as to the numbers made. Crecy engine #10, achieved the highest test horsepower in December 1944, putting out the equivalent of 5,000 brake horsepower.