In reply to Kemics:
Well, I've got SPDs on an MTB, a fixie and a road bike. My shoes were a bit past it and so there was much humming and ha-ing. I've got one spare pair of SPD-SL pedals, but no cleats, so I could have bought some road shoes and SL cleats and made do with the old shoes for the MTB and fixie, and then bought another pair of SL pedals in due course for the fixie.
However, the fixie's a commuter and so that's not brilliant.
And - whatever my problem is, it's not being unable to put down enough power through SPD pedals. So I bought a very nice pair of new MTB shoes with carbon soles and stuff.
Shame I bust them after two days. Waiting for a replacement pair now.