You could do that, but my concern is that simply laying a coil on the grate isn't as much focused, intense heat as placing it into a chimney with red hot coals or a burner with direct heat. The burner on a propane grill is probably much lower BTU's than a turkey fryer burner, too...
True, I guess it depends on what's more important - efficiency of propane or speed. If you're looking to multi-task and cook food at the same time, you can throttle back the water speed and while it will take longer than the chimney column, it's may end up faster than the stove or full pot on the burner and still cook using propane you were using for the food anyways, saving you some $.
if you want super fast super efficient - then yes, I see your point. All depends on what you want out of it.
Since we're comparing this to cooling products, I see potential in a reverse plate chiller. You blast major heat on the many many plates while pushing liquid through, and voila - it sits nice and flat and might be ultra efficient if you encase it in some sort of box maybe?
It would be interesting to at least test if anyone has a plate chiller that is deemed not safe for wort but can still be used as a test device to see how fast water can heat up in it.