I shot this 4 hours ago with the same exact combination (5D Mk III, 2x Extender Mk III, 70-200 II). I cropped it all the way down to 1.6 Megapixels 300dpi and did some sharpening to it (maybe a little too much sharpening). I like this camera/extender/lens combination.
Can you control the Canon 600EX-RT with the new PocketWizard Plus III Transceiver attached to the camera's hot shoe, without the need of attaching a Pocket Wizard to the 600EX-RT? Or do I still need a Pocket Wizard on the 600EX-RT even though it uses radio signals?
V8Beast, could it be that you have highlight tone priority, and/or auto lighting optimizer settings enabled? If so, try shooting with them disabled That might be the cause of your under exposure problem.