Originally Posted by Cari
Sometimes people with wide feet get used to going up a size to accomodate the extra width. If this is the case, then maybe you're tightening the straps too much in order to keep your heel in place, and thus reducing circulation to your toes. In addition to that, if one of your shoes is a tad too long, then when your foot starts to sweat, it can slide forwards and backwards inside the shoe, which can also hurt your toes. Or the heel slip thing might just be because it's a men's shoe, which has a wider heel than a women's shoe.
Are your current Sidis in a full size, or a half size? The sole on half sizes is the same length as that of the corresponding half size, but there's more fabric on the upper -- similar to the Mega shoes, but to a lesser degree. So if you're using a full size, I suggest trying the half sizes above and below your current size as an alternative to a Mega. Since you said that you overpronate, I'm guessing that you might have low arches, so the extra material on a half size might be enough to accomodate the extra width.
In fact, I would even suggest that you try a women's half size. I have wide feet too, with low arches, and what worked best for me was going up a half size. My old Sidis were men's 41's and like you, I had problems with both toe numbness and heel slippage. I would wear thick socks so that my heel wouldn't slip out and my big toe wouldn't jam against the front of the shoe, but the shoe felt narrow in the toebox, to the point that towards the end of a long ride, it would go from numbness to a burning sensation. My new Sidis are women's 41.5 and they fit *much* better than the men's 41 or 42 (which were huge). They actually feel wider in the front than the men's 41, but narrower in the back so that it cradles my heel in place. I still wear the thick socks, but now the shoes fit like a glove.
Good luck with your search, and please let us know how it turns out.