Well the Italians did send 200 aircraft to the BoB so the effect on other fronts would be the same as OTL. The Italian CR 42 biplanes found they could easily get on the tail of a Hurricane by a single turn and didn't do as badly as you might expect. If they were mixed up with German formations then the Germans might have suffered fewer losses and even if they were on their own, if flying at the same time as German attacks, sheer weight of numbers might have protected them. The Italians would have followed their own strategy so perhaps they would have attacked targests that were part of the Sealion plan instead of Goering's insistance on doing it his way and ignoring naval targets.
Assuming the Italians were ready in the middle of august with these 200 planes, then I suppose its about how close to a tipping point were the Germans in the BOB??? The tipping point where the British change their strategy, pull back fighter command behind London and try to reduce losses for the day Sea Lion comes. If the extra planes cause this, then something worthwhile might have been achieved.
(I suppose the Italians were banking on the Germans winning the BOB, forcing some peace and then they get in on the peace negotioations and get stuff for cheap. Seems silly in hindsight but maybe not then)
It does seems 200 aircraft would have been useful in Greece, suppressing Malta, etc...