This matches up exactly with a test of R2054 I have, and I don't think this would be a 'representative' Mk.I (though I'm far from any sort of Beaufighter expert).
R2054 was noted to have:
- early, prototype nacelles
- early, poor finish
- no flush riveting
- external escape door
- retractable tail wheel
I think a better bet would be R2060, which had did not have any of the above (and so had flush riveting, though no retractable tailwheel). This Mk.I was compared to the older one and did 323 mph at 14,250.
EDIT: Also, X7540 and X7542 were tested at 335 and 333 mph respectively, both just under 16,000 ft. These had:
- Hercules X
- improved air intakes
- undercarriage door close mod.
All this said -- the Typhoon had 55+ to 75+ mph on the Beaufighter under 10,000 (depending on the number of Typhoon refinements).
For what it's worth, in British testing there was almost nothing to choose speed-wise between a Boston III/IIIa and a Beaufighter (Herc X/Herc VI).