The amount of energy in the exhaust is not insignificant....considering a typical efficiency of a internal combustion engine is <30%, most of that energy goes into heat, and a good chunk of that I'd think is in the exhaust flow....
The current F1 shouting war on "blown diffuser" is basically a way to scavenge(and actually using the exhaust by actually not just scavenge, but actually using the hot exhaust) to drive the diffuser flow....
University of Toronto Formula SAE Alumni 2003-2007
Formula Student Championship 2003, 2005, 2006
www.fsae.utoronto.ca