In conversation with my brother who is building the replica in his garage, I was somewhat taken aback to learn that a development note he has been reading firmly stated that an aircraft on test with maximum boost and 100 octane fuel, which was fitted with Rotol propellers  with an  increased coarse pitch capability compared to the DH items managed 411mph in level flight and enhanced altitude performance, and this could easily have been combined with the lower drag more efficient Morris radiators. The modified four cannon nose also contained an auxiliary fuel tank that would have extended its range. It seems performance could have been boosted considerably at little cost but it was never approved for introduction sadly.