Footing the end of the shaft I don't think will make it that wider. From what I've seen of some of the steel broadhead adapters and broadheads, they are slightly larger than a 5/16" shaft, so adding another thin piece of aluminum on top in my thoughts shouldn't hinder it as much as having it plain and chancing the breaking behind the ferrule. I mean, they sell 23/64" shafts, do the broadhead adapters and broadheads not fit these also?
Isn't the broad head blades wider than the ferrule, once it's past the end of the blades, it's up to momentum and force anyway. The only logical thing I can see that would help you is skinny carbons with the small diameter shafts to reduce the friction.
I'm all for getting the most out of an arrow, but, it comes down to what I shoot the best for me. I've been using small carbons front loaded heavy for FOC, and I like the way they shoot and respond. Under any stress an arrow will break, one way or the other. Even an arrow striking a hard object or shoulder bone can break with a steel insert. Steel is still harder than any arrow we shoot, if the broadhead buries itself up and the arrow is still flexing around it, it still has a good chance to snap off and break. More likely with aluminum than steel? Yes, I'll concede that. But I've never had it happen with aluminum or carbon arrows, I've always had the insert shoved back into the ferrule. Structural integerity is important, do not get me wrong, but, I belive tuning of the equipment is more of a affecting influence than the makeup of the insert is. That's just me though. However, if I were to go after larger game than what is native to North America, then I would concern myself with the front end, and probably reinforce it then.