I disagree, the canon heating, anti jamming problem was resolved way before the 20 series was produced. Infact the Hurricane and Spitfire had these freezing/gun jamming problems as early as the mk 1 browning .303 machine guns but was sorted out pretty quickly. So im not sure where this information is coming from.....
Plus the Hispano 20mm cannon were actually very reliable. From the Mk V onwards.
From everything I've read from the powers that be themselves it was due to the second issue (performance penalty).