I haven't but would think it would be minimal, probably less than having an extra average size adult on the boat! Once on plane it shouldn't be an issue unless your pulling a "Barge Size Load"
We do not toe one but have met some people that do but not in Searays. The comment from one with a 16footer with a 50 on it was it slows them down about 10% at same RPM.
It really depends on the type of dinghy and how you tow it. If it's a hard bottom dinghy, they don't have too much drag, but will cost you some (less than 10%?). We have a hard-inflatable, and we don't tow it anymore - too hard and too much drag.
For long trips, we take the engine off and put it on board. We spin the dinghy around backwards, lift up the transom, and tie in onto our boat on the big boat's transom. There is almost no performance loss when we do this. For short trips, with lift the bow of the dinghy onto the transom of our boat - not too much drag here, but no numbers to confirm.
Your boat is already a heavy boat for the hull size, so, I would think you would barely notice the dinghy back there. The hard part is not tearing the dinghy up while towing it at 25 mph.
Thanks for the info. I am looking at the Mercury 240 Airdeck which I think I would tow or stow. I have been told every thing from a slight drop in MPG to a significant drop.