RockyTop,
The ORECA wing (McCann Racing, Archer Racing) looks like it mounts to the rear deck but it actually bolts through the body panel to an aluminum bracket that is mounted to a rear structural component of the car. This is the carbon fiber one most of the VRL guys are running. I believe the older style ORECA wing (Autoform, TNT, Archer?) also bolts through the rear body panel to a structural support. As does the dual plane wing from Parts Rack.
Wings that generate real amounts of downforce will deflect the body panel and do all sorts of nasty things to the handling of the car.
Regards,