Having been professionally employed as an aircraft performance aerodynamicist, I can assure you that an aircraft’s best turning performance can be calculated, or indeed its actual turning performance at any combination of speed, altitude and climb/descent rate. Yes, actual combat had many variables, but comparisons at the same conditions are still valid.

Deighton’s figures were lifted from Mason’s Hurricane Mk.1 Profile, which assumed all three aircraft had a maximum lift coefficient of 1 – which is nonsense. So please do not quote it as relevant information. The matter was readdressed in the 1990s in the Royal Aero Society Journal, and an article produced by the staff of Manchester University. Now where did I put it…..
From memory, the Hurricane had the best turning performance and the 109 the worst.

As for the Hurricane’s kills: yes the Spitfire did fly in most of the theatres that the Hurricane did, but it was kept “on the island” for so long that (generally) by the time significant numbers of Spitfires saw service overseas, the enemy was totally outnumbered by other Alllied fighters wanting their share of the hunting, and opportunities were less than for the earlier Hurricane. As indeed were losses.

I’m not convinced that the Hurricane did achieve more claims than any other RAF fighter – dragging in other Allied fighters seem to be going beyond the original suggestion – but it is not obviously silly.