However, given those extra pounds, I am like a bat out of hell on the downhills.
Me too!
I had some pretty bad hot-foot with my Shiman SPD cleats and a couple of things have helped - one was getting some inserts from REI - that have some that are designed for cycling - I think they are color-coded - maybe the blue ones are for cycling. The other thing was to keep my shoes pretty loose. The spd shoes were like yours - mountain shoses with recessed cleats.
But what really helped was switching to full-on road shoes with Looks, which have a large platform. Good luck!
Keep calm and carry on...