I'm assuming these are 3 bolt shoes? I've been able to get an extra couple of mm of adjustment range by filing the rather large button heads on Shimano SPD-SL bolts. Did so to get more aft on a pair of previous shoes I've tried.
Talking about these bolts:
According to my cheapo calipers the heads are just over 14mm diameter. The hex cavity is a tick over 4mm so you can probably safely remove 5mm around the outer diameter and still have enough head surface area to tighten them down. The ones I hand filed I removed around 2-3mm and it worked fine. Cleats were secure and never moved.
I did the material removable by hand and worked the heads against a file....it sucked. If I ever have to do it again I'd rig it up to be able to take my dremel tool with a grinding wheel to do the work instead.