The Plus 8 not only dirties its own nest, You will also inhale the acrid smell of exhaust fumes every time You accelerate. I usually hold my breath for the first 5 seconds when accelerating....
Peter - perhaps you need one of these on the back?
If Mark is correct that might be closer to the answer than you think!
I remember in the long time past that hot hatches that had a spoiler below the rear window had clearer back windows than ones without so what you're probably getting is all thrown up from the back rather than from the front.
Haven't noticed fumes in Aero 8, but in really bad weather you do get some muck on inside of screen and mirror, but not so bad on instruments. The Aero windscreen is a much more efficient 'windscreen' in my experience than on the Plus 8 which was rather buffeting in comparison at, say, 60mph and I guess will cause more grime to come in. Design of doors and rear end very different and I doubt the Plus 8 had much attempt at aero (more a hoped for way of shifting a few more 4.8l engines that were lying about...) Interesting and perhaps the car's way of saying (don't drive me in such nasty conditions - I'm a tart really!)