It's all right at 110. I've had it up to 120 and it was fine at that speed too. Really 100 is best for my car, it starts to run out of puff much above that. I've got the sports suspension which lowers the car by 20mm. This probably enhances high speed stability, too. I was on a very well made stretch of road on both occasions: that helps more than anything!
I know there is a top speed thread somewhere, but my best is an indicated 160 in an Audi A8 4.2 quattro; my dad's BMW 728 would do 145, his 996 is just getting started at 130 but he's quite precious about it, and my mum's SLK230 will do 140 with the roof down... The SLK is an unpleasant car at high speeds, it's handling is very pointy, perhaps because of the short wheelbase and wide track.
What's the S420 like? and what is your mum's Porsche?