I'm sure it might be done for cool points, but the exhaust flow must be pretty insignificant compared to the air displaced by and moving over a (street) car. And unless the engine's going to keep pushing air when you let off the throttle, that little extra downforce goes away when you actually need it.
And I'm still calling this USA NSX a Corvair.
And I'm still calling this USA NSX a Corvair.
The only thing that stops a bad guy with a van is a good guy with a van