Doesn't really matter what the range is if the thing is a dog in combat.
The only chance it had at being a single seat fighter was in 1939, before the first ones were issued. By the summer of 1940 when the Prototype single seater flew it was a dated design.
Much is made of the "projected" 360mph top speed. This was "projected" using a Merlin XX engine at 21,700ft. Also projected was the ability to out climb a Hurricane IIc at sea level by about 500ft/min and be able to climb to 25,000ft in little more time than the Hurricane IIc took to get to 20,000ft.
All this while carrying 4 20mm cannon AND four .303s AND the 20mm guns could pivot downwards by 17 degrees for ground strafing.
As noted already, Production MK II Defiant's using the same MK XX engine gained a whopping 10-12mph over the MK Is.
The Spitfire with drop tanks wasn't going to be a long range fighter but at least it COULD fight in the areas it could reach.
This single seat Defiant wouldn't reach service use (with the MK XX engine) until the spring/summer of 1941.
While a bit later in timing the P-40F makes a good reality check, A bit smaller wing, a smaller fuselage, lighter weight armament, Fuel load??? and goes 360mph at 20,000ft with the same engine.
I've already stated that CBI and MTO would be the primary battlefields, Italian and Japanese fighters would not have the edge to put the single-seater Defiant into a 'dog' category. The armament set-up proposed by BP was unrealistic, fixed 8-10 LMGs would do the job, or 2 cannons + 4 LMGs. The projected performance figures are also a bit too high, wing of P-40 was also thinner. With historical Defiant II making, what, 315 mph, a similar plane minus turret, but plus fixed guns, would gain maybe 10-15 mph. Comparable with Martlet/Wildcat, or Hurri II?