According to what I have read, the Malta Spitfire Mk Vc' had their 3rd and 4th cannons removed. This was done for two reasons. One, two cannon were found to be more than adequate firepower and the extra two did more to reduce the Spitfire's performance than to increase the Spitfire's leathality. Two, 20m cannon ammo was not in plentiful supply on Malta and two cannon fighters use ammo at, well, half the rate of four cannon fighters.
I don't think this option is a good idea in AH. Our accuracy makes firepower the dominant trait of fighters (just look at the F4U-1C vs the F4U-1D). Adding this option would basically eliminate the common Spitfire armaments of two 20mm cannon and four .303 Brownings or two .50 Brownings. Lets keep flying the Spit with its most representative armaments.
Besides, its already pretty easy to get kills with it.
