Plan on opening up the rear grill on my ST as well to reduce parachuting (car basically feels like I hit a wall at ~135mph on the straights). Already did the front grill years ago for DHM FMIC, so I know the joys of sanding/melting the plastic.
However, since my ST is ~90% track duty now, I am contemplating just removing the whole rear valance. Sure, it may look funny and I would lose a few mounting points and some structure, but maybe it would be a wash with cutting a lot of air packing/drag.
Thoughts?