I had the same problem with my Cannondale M600. My heels were hitting the panniers which could not be moved further to the back because the external pocket at the back of the panniers bounce into the spokes when I was pedalling heavily and wagging the bike from side to side like when stnding out of the sadle in a climb. I made a pair of extentions for the rack to mount further back in the small triangle. Welcome to take a look

http://www.karlstam.com/

Just klick on "Bicycling" in top main menu and then on the link "Pictures and tips about the bike and gear". The pic is not the best but you get the picture?

Regards
Anders