My feet go numb too, especially one of them. Funny thing (and thankfully!) this never happens in my winter mtb shoes. They have a very stiff sole, are roomy and I wear a wool insole in them. So I can't just fault the spd-pedals, it's my summer mtb shoes that are too soft, too easily tightened or something. I can get numb feet in my roadie shoes with Keo pedals too, but that's usually when I tighten the shoe too much.
No real advice, except try looser shoes, different insoles, socks etc. It's not necessarily the pedal.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett