I know people who have ripped out of mtb SPDs in sprints, so I went with Look pedals/cleats. Road shoes are much less fun to walk in, though.

there are some specialty CX shoes, with really stiff soles, but you can still walk in them. I'm not sure what your cleat choices are, but i'd go with eggbeaters if possible. Just mash in, and the more you pull up, the more they tighten down.

my $0.02