To be honest, if you are in a standard width Sidi and find it the most comfortable, then going to a mega Sidi will be to large. I would suggest going to the next half size up, so 42.5 as it would give you a little more volume and a little more width, which is what you seems to be after. You already said that Sidi is the most comfortable, stick with what works.
I suspect as he stated:
the straps are only strapped halfway to 2/3s of the way before they get uncomfortable for me
That he has quite a wide foot.
Just to clarify, geedawg, have you tried any of those shoes in wide? From your post it sounds like you're just using the regular (medium) width models of all those shoes. I know that all of those companies do make wider versions but you are unlikely to find those in local shops. To mm2zz's point you might want to track down some Mega variety SIDIs.
Of course if you've already done all this and you're running out of options, forgive me.