I wonder if the one you drove maybe had tires that were over-inflated?
Its been a long time since I've driven a 5, E class, or an A6 but the GS350 I drove surprised me by how comfortable and well damped the ride was given things that I had heard. I would say it was almost as smooth as my ES...
As an aside, I stopped by my dealer this afternoon to get some new wiper inserts for the ES and I sat in a couple. I have begun to *really* like the car. I sat in one with the "Flaxen" interior which I originally didn't care for, but I really liked it in person. Very different which is a plus, I liked the corresponding flaxen colored stitching to the dash top as well as the steering wheel, shift knob, and shift boot (which are all black). I also sat in one with the grey interior which was also very sharp.
If I didn't know I'd take it up the pooper so bad on the ES I'd be tempted. Only bummer for me is the car equipped how I would equip it doesn't have the luxury package, and that car has a matte wood trim and no wood/leather wheel like I'm used to. You have to move up to the luxury package to get glossy trim and a wood/leather wheel and that makes a $55k car a $61k car which is more than I want to spend. It would take some getting used to...that and theres a pretty significant rear seat room penalty over the ES, which can be an issue for me from time to time.