Baz,
my old car, with a much younger me at the wheel
Big 'ol live sidepipes.
I'm not saying don't do it, clearly it works for some cars, and the shape of the hood would have lot to do with the aero effects.
The good news is the leather you have in the car kept a few cows dry during the winter months, so it should hold up great to anything you throw at it. I have really good leather seats in the current car, they have been rained on and roundly abused. A good clean and leather conditioner and they look showroom...
Cameron