OK, so I ran a real short test.
F4U-1C vs A6M5-52 ace, Okinawa, 1000m, all as prior. Did my lil barrel slide to get past and leveled out about 1400 ft. 52" MAP was right at 99% throttle, 100 pitch was 2750rpmish, rad at 40%. I ended up going about 240-250 KIAS, which is roughly 280-290MPH as you said. I sloooowwwwly pulled away from the zero. Maybe .01 km per sec. Decided to put it in a shallow climb like the AI does, kept MAP and RPM at 52 and 2700, rad at 40. When I hit about 5000 ft the MAP dropped to 50 so I shoved it to low blower and reset for 53" MAP, which turned out to be about 95-96% throttle, pitch remained the same and 2700 rpm on the dot. The entire time I didn't touch the rad but temp kept creeping up to where the gauge was about 1/8 below full mark, dunno what that temp is (didn't zoom in to look).. Also, the kicker is, my speed dropped to about 200 KIAS and wouldn't go any higher, and the zero started gaining on me rather steadily. Kept it straight and level but still nada, wouldn't go above 200 KIAS. Engine sounded normal and strong, the temps were high but no overheating message, so unless damage or power loss occurs without the OVERHEAT message I'm not sure what happened and why it wouldn't accelerate.