Martin, you'll be wasting your money!
Whenever I hear car 'audiophiles' boasting about how good their system sounds, I always wonder if they have ever heard a real home setup.
My freind has a brand new Lexus LS430, which has reputedly one of the best car systems, but although it sounds good, I wasn't impressed. Still nowhere near as good as a proper home system. The car's main advantage is that it is very, very quiet when being driven.
But anyone can significantly improve the standard system of course simply by adding a sub. It's the low frequencies which suffer most. You just need to overcome the cabin noise with a decent amount of low distortion amplification.
After that, the law of diminishing returns kicks in. You'll notice a difference between the standard system and an £800 setup, but you probably wont notice a difference between an £800 and £2000 setup.