This is a great idea.I'm sure it is an old idea,but I'm guessing it is now feasible because there are now tiny turbines with very low resistance bearings.
Prior to hybrids becoming popular the only good way to capture wasted exhaust energy was to use it to turbocharge a small engine,and make it act like a "big engine".Unfortunately spark ignition engines can't really benefit much from turbocharging;they are very prone to detonation at over 5 PSI boost.With a Hybrid the wasted gas velocity can now be used to charge the batteries and run the electric motors.6KW is about 8 hp.8 HP could probably push a Prius at a steady 30 mph.
A Gasoline Hyrid equipped with this gadget could probably match a TDI Hybrid in mpg,and it would be cheaper and lighter than a TDI hybrid.
Back pressure shouldn't be a problem.If there is a bypass used it won't increase backpressure at all,and you could probably decrease muffler weight and volume since the turbine will drop the sound energy a lot.Great.Charlie