I don't think they add too much performance on the xfire as that rear section is already shaped and works like a diffuser (gradual rising slope) but there are probably some benefits when getting closer to top speed. If we had a full flat panel underneath they may work a lot better. Basically they just channel the low pressure accelerated air from under the car out the back end in a way that allows it to decelerate/decompress smoothly. This should increase downforce if done correctly (from my understanding) the little vertical pieces are mostly there to prevent turbulent/slower moving air from getting in the sides and mixing in with the low pressure stuff. In this application they are not really long enough to do so (they would need to extend well under the center of the car) so they just help smooth out / direct airflow.

If I remember correctly it took them a while to get the rear end designed right to be stable at speeds. The mopar piece basically just adds strakes to what is already essentially a diffuser built into the back.