So here are a few more from Pacific Victory Roll, right up to the end of the war.
I don't see this as such a surprise, because the Beaufighter is fairly fast down low near sea level where it operated, and I think competitive with the A6M on speed at that altitude.
This chart shows a Beau Mk VI at 312 mph TAS at 4,000 ft, and that is at +8 lb boost. I'm not sure precisely the speed of the A6M2 or -3 at that altitude but Allied tests seem to indicate it's slower than that. For example
this test shows an A6M3 at 280 mph at Sea Level. I know there were some problems with these tests so I can't be sure how accurate that is..
I think the key for a heavy fighter to have success against single engined types is to have a substantial speed advantage. That way it can hit and run. As soon as the single-engined types get to parity or better, the heavy fighter is no longer successful, and it's a pretty fast tipping point when the speeds change. This worked for example for the Bf 110 in the Battle of France and early days on the Russian Front, but by the BoB and early engagements with Hurricanes and Tomahawks in the Med it was no longer successful at an acceptable rate.
In the Pacific, a Beaufighter I think was a bit faster than an A6M2 or Ki-43 down at low altitude, and certainly could catch float planes and bombers operating down low.
in the Med, a Bf 109 or MC 202 had such a speed advantage that the Beau was more challenged.