I've read also that the guns of a spitfire had different convergence for the different guns. Although I understand it would be good for the pilot to see his round hit, it also makes his guns less effective.
Also the spitfire couldn't get the convergence as close as the pilots would like. I believe the minimum distance for the the inner guns was 180m. While dogfighting took place at much closer ranger (80-150m, some even closer, check Hans-Joachim Marseille and ofcourse Erich Hartmann as close as 20m ), even British aces hated that long convergence range of the spit (can't find my source again). However the factory standard was a whopping 350m (some Osprey book I read)
Therefore I think that the 20mm + 2x 7.9mm armament of a BF109 is much more deadly than the 8 0.30 of a British early war fighter. Even late war the above mentioned armament can keep his own against wingmounted 20mm's