The ride on my GTI (17" wheels) is the only bad feature of the car, which is brilliant otherwise. It's especially bad on concrete motorway sections where the lateral ribbing makes the car lurch, as described. And on poor roads (most of them!) it feels hard and bouncy, with no give.
I read an letter in Volkswagen Driver (Dec 2006, page 52) whilst at the dealer yesterday, where a GTI owner talks about not being able to stand the harsh ride and upgraded shocks to Koni FSD. These apparently preserve or even improve the handling, and at the same time make the ride very comfortable. I'm seriously thinking of doing the same...