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Running AC off the exhaust is not a good idea, you'd be subject to variable amounts of heat available and variable temperatures, aka your AC isn't really going to work most of the time. Belt driven wastes power by overrunning the compressor but at least the compressor has adequate power all the time and functions.
Exhaust driven turbines I think are a good idea if the idea is to act like a muffler that returns some energy back to the system, but they also don't generate much power and cost too much money for what they accomplish. Better to just save the energy upstream at the engine using Atkinson cycle. At maximum power like F1 most of the time, it's useful because then the turbine produces a significant amount of power and raises the efficiency of the engine appreciably.
I think exhaust heat regeneration devices that actually come on cars in the near future will be Seebeck effect (thermoelectric) devices, once they figure out some efficiency improvements, or similar very high power to weight ratio heat engines that don't rely on a gas turbine. To condense a low boiling point fluid and drive a turbine would require a low temperature condenser which is going to be difficult. If they figure out some nice 800C working temperature thermoelectric devices that are cheap (not even more efficient), you can run 80C engine coolant through the cold side and end up with enough power for the car's basic electrical demands under cruising conditions, with pretty little effort.
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