[AtomicLabMonkey]

The extractor might help quite a bit if you haven't run with it at speed yet. As far as other aero tweaks, I dunno if you just left it off but the pic only showed one side sealed from the bodywork opening to the rad. There are other things you can do to help vent air out of the engine bay; massaging the bodywork behind the tire as illustrated can help quite a bit. Keep in mind that anything you do to lower pressure under the hood will also act to cancel front lift/create front downforce, so it will exaggerate any current high speed oversteer tendencies. A fancy wing also usually isn't necessary to create some actual downforce on the rear end to balance things out - a NASCAR style spoiler is cheap, very simple, and effective. Might not be the look you want, but it works.

The other side of the equation is the coolant - I don't know what options are available, but have you looked into higher flow rate water pumps? Pushing more water through will work just as well if not better than pushing more air through the rad. Or maybe the current pump is starting to cavitate at high RPMs and needs to actually be slowed down? I know that would happen with the power steering pump in particular on the Mustangs. I dunno, things to think about.