How about sending the exhaust into a circular chamber with a spinning wheel at the end to slow the exhaust down and absorb some of the energy? Of course the chamber would have to be very strong, and the wheel would have to have very good bearings and be made of something like inconel. Perhaps you could even harness some of the energy captured by the wheel to do useful work....

Hey! Hold on a minute.... there's such a device on the market - A Turbonetics turbo. :)

John (quieter than a C172) Slade