As it stands now, I don't want to see 4x20mm on Spitfire. Not because it will unbalance arena (which I doubt), but simply because it was such a rare thing. The least we have of those uncommon loadouts on planes, the least chance we see people asking for adding some unusual/testing weapons. We have enough of wars with which plane to model, let's not start another one.
In addition, that loadout won't add anything significant to RAF arsenal, SpitIX is outdated in MA, and there is no reason to beef it up just to make it "more competative".
mx22