In reply to RKernan:
I also have wide, flat feet, with long toes where my little toe is almost as long as my big toe, so a wide forefoot. I am a UK9, and wear a UK8 anasazi (but only because I can't get anything smaller past three smaller toes).
I have a pair of UK7.5 Instinct Laces, and I think they are half a size too big. UK7's would be suitable.I suggest that it may be that you're fitting them wrongly. You say you do not want to go below a 9, but there is no way that can fit you well. Whilst everyone's feet are different, I can get 7.5's on with bridgedale wool hiking socks on.
If I might be so bolt, I suggest to you that you try the smaller sizes. In my experience, heel rands will cram your toes into the end of a shoe whether or not you are the right size for it. All that happens if you go down sizes is NOT that you go further forward into the toe, but that your heel begins progressively to fill up the heel pocket (as in your foot moves relatively backwards in the shoe). This does have the effect of tightening the heel rand a little, but then the shoe fits you.
However, if you do not want your toes to be curled in a shoe at all, and want a flat foot profile, then it might be an idea to rethink the choice of shoe - maybe something like a 5.10 Galileo or Evolv Bandit might suit better as that has stiffness from the midsole, and doesn't require curled toes as much.
To answer your original question; it depends on your foot. My 7.5's are unlikely to stretch anywhere but across the arch where I overtighten them. If I got 7's I might get a fifth to a quarter of a size worth of stretch in the toebox, but it is mostly covered with rubber.
Sorry if that is unhelpful.