I'm not sure if my head is proportionally as wide as yours, but some years ago when I used to ride horses I had a major helmet width problem there. Any helmet that fit my head side-to-side was so long front-to-back that it would tip forward and back as I rode, and the alternating thumping was rather unpleasant. So I had to ride wearing this thing that looked like a chamber pot. *sigh*
Anyway, I recently got a Met Ippogriffo and find it really comfortable (I got a Large 59-61cm which was fine for me but might not be big enough for you; there's no Extra Large as far as I know, unfortunately). Met's Stradivarius model fitted and felt exactly the same, but I didn't like its looks as much (and it was about AU$70 more!). They both have an adjustable inner, um, cradle I suppose you'd call it, that goes all the way around, so your head doesn't get squidged up against a particular part of the helmet (which I noticed on some other brands I tried).
I can't compare it to the Bell Ghisallo or the Giro Pneumo (the two I
really wanted to try, although judging by your post I can stop being frustrated about that now
) because they're pretty difficult to get in Australia. I'm not sure what the situation is with the Ghisallo, but with the Pneumo the Australian market is apparently too small for Giro to have bothered submitting it for testing, so it's not approved for general use here; I know one place I could get one mail order, but I haven't seen either in any of the many bike shops I've been to.