I guess I've always thought of the rear of our cars as having quite a parachute affect in comparison to more modern cars with undertrays so maybe giving the air a bit more of an escape path whilest helping cool the exhaust is the way to go.
On top of this the plate above the exhaust is a whole lot more replacable if it all goes wrong or doesn't work. Drilling the rear crossmember is a bit less reversable!
What I'll do is put some holes through the panel and some heat reflective material on the exhaust side.
The other smaller vent will probably be adding a grill/mesh where the licence plate sits and mounting the licence plate onto this. Hopefully the plate won't melt or go up in smoke at the first set of traffic lights!!
