On most unibody cars, the rear quarter panel and fender is kind of a key structural component. On a race car with a great cage, pretty much the entire body becomes decoration, as the cage is tied closely into the suspension and takes most of the chassis forces. So, it’s not such a big deal that a big chunk of the original panel is missing.
I think a lot of times people run to the vent shop when they might need to do something else, instead (ducting, under tray, etc). It’s also important to make sure that the vents are located in a low pressure zone. The reverse-facing louvers might cause enough air turbulence to create negative pressure, but I’m no fluid dynamicist.
It looks cool, either way. Pun intended, I suppose.
That rounds out the aerodynamic modifications. But, Solo Motorsports let it slip that they have a completely redesigned aero package in the works. I’m eager to see it.