I was out at DPA last week and spotted either a C172 or C182 with a canard installed. Would there be that much of a performance increase with them?

Picture? I did not know such a thing exists, but at any rate it would have to be a fixed forward stab, not a real canard. If it does exist they would have to trim it for cruise and the only advantage would be unloading the tail stab to reduce the downward force. This would be optimized at forward CG, and if this airplane is used commercially in a cruise mode a lot and is loaded forward CG all the time, then it might produce a small advantage in efficiency. Slightly higher cruise speed, less drag, better range, etc. not a lot. You can get the same deltas comparing performance of a fwd CG loading to an aft loading in an unaltered model. There is also a small drag penalty for additional wetted area with the fwd stab, and some extra weight. That plus cost, and the structural issues are why these are not common. Also degrades stall-spin recovery a bit.

As I understand it, the canard is there to provide better behavior during slow flight for STOL operations.

Check this out:
http://www.katmai-260se.com/features.html

The Katmai mod has won numerous bush plane "shootouts" in articles I have read. Supposedly it can go head to head with the big contenders while carrying 4 pax and gear.

The plane
http://www.planeandpilotmag.com/airc...e.html?start=1

That looks like a true canard. They are charging a lot for a slight decrease in stall speed and controllability, $25k for the basic canard. I think I would just get some VGs and glue them on. I guess the argument is it makes the airplane slightly more predictable near stall. But you also take a 5 knot penalty in cruise due to additional wetted area, dirty flow to the tail, and a small hit to useful load.

Looks like the old Wren mod:Peterson (Wren) modification of Cessna 182

I have a few hours in a Peterson....it was something....off the ground in no time and FAST.