Hi Tony,
>If you could design one additional aircraft for the RAF/FAA, to be in full squadron service by 1940, what would it look like?
Interesting thought experiment! :-)
The the Gloster G.39 the same as the F5/37? It seems its primary engine choice were Peregrines, just as for the the Whirlwind. (Are you certain that the 360 mph top speed is for the Taurus variant?)
I don't know much about the Taurus engine, but from the Wikipedia article it doesn't seem like a competetive fighter engine with 1065 HP at 5000 ft for 5 min power.
Higher octane fuel would give higher take-off power, but at combat altitude the Taurus would still be weak. Adding a more effective supercharger could raise full-throttle height only at the expense of shaft power, so you'd end up with a poor fighter engine anyway.
For carrier-based ground attack operations, on the other hand, the Taurus might be an excellent choice. It appears that the Seafire was predominantly equipped with low-altitude rated Merlins similar in altitude characterstics to the Taurus.
With regard to the long-range air superiority fighter, I don't believe that role could have been played by a carrier-capable twin in 1940. With the shortcomings of the Taurus, I'd rather try to get a single-engined, Merlin-powered long-range fighter into service - less weight, less drag, less production strain. Hindsight regarding the success of the Mustang validates this idea ;-)
As a bomber destroyer, a carrier-capable twin might be feasible, but again, not with Tauri. (I think you're probably underestimating the weight impact of designing an aircraft for carrier operations, but even a relatively heavy type like the Me 110 would have helped the RAF greatly in the Battle of Britain.)
For a carrier-capable twin, I'd suggest an aircraft laid out like the Heinkel He 219, but smaller and considerably lighter. The tricycle gear appears like a must for safe carrier operations, and it will also help in short-field landings. High-lift devices will be required for short-field and carrier take-offs in any case. Until the Hercules comes along, the only British engine I spontaneously can think of as competitive is the Merlin, so it will be inline-engined.
Without the benefit of hindsight, I'd have been tempted to suggest a lengthened, drop-tank equipped, Vulture-engined Whirlwind variant, but knowing the fate of the Vulture, I'll stay of that one :-)
Regards,
Henning (HoHun)