I find in ultimate heavy downpours all shoe covers I've tried have a failure point where your feet just get soaked. My MECs give up completely after about an hour and a half, which is really not too bad. So, on really long rides in the winter I wear two layers of booties, the MECS on top and some lighter, lower profile roadie booties underneath. This is good for a bit longer, although nothing's perfect.

I ride clipless btw, you toeclippers might look in to rubber overshoes and powergrips for really cold winter rain. Whatever you get you'd just want to make sure it can take the added wear over the arch.