Having known people who have flown with leather helmets (B and C-types) I can say for a fact that as soon as radio is involved (and therefore earcups, doughnuts and receivers) ballooning was a fact of live.
Certainly with the increasing speeds, open cockpits were no longer viable. Not only because of safety and comfort, but also because of the impossibility of practical communications.
Even with Gosport tubes, comms were difficult to say the least. Again because of the ballooning affect.
So chris, although esthetically maybe not desirable, adding doughnuts and having the sides of the helmet bellowing out would certainly be the right look!