Shortround6
Major General
Further on the subject: the RAF was fielding the BP Defiant, certainly not that cheap/simple as Hurricane, or either Spitfire, with it's turret, greater overall dimensions weight. It was also feturing fully enclosed main U/C. One of the planes with enclosed U/C were the Macchi fighters, too.
As for planes with high speed achieved on modest power: Yak-3, He-100, P-51 51A.
Solvable by better flap system?
He-100 featured also the wing of modest area, and it was fast, even with 'classic' cooling. About P-51, well, much is already written.
Of your 3 examples 2 carried a rather modest armament. One might go so far as to say their armament was crap. None of them were combat capable in 1940.
In 1940 the British, for an airplane produced in large numbers, are pretty much stuck with 6-8 .303 machine guns. Given that the choices become some what limited. HE 100 might carry 5, you just need to redesign and enlarge the wing root. Yak can carry 3 without sticking them in the wing.
Fully enclosed landing gear certainly wasn't unheard of or unused, but it is one more complication. And you don't really know what it is going to do until you test it as you don't know what the local airflow is on any given airframe until you do test. You can't say Plane X picked up 8mph therefore plane Y will pick up 8mph.