More speed definitely helps, although I'm not familiar enough with the 407, having only driven it for about half an hour, albeit on a mixed bag of roads in Sydney. The 605 becomes really compliant once you pass about 100km/h. Heading onto the Harbour Bridge from the city, the lanes are full of cracks, filler, old lane markings, etc. These are not upsetting at all but can be felt, (and are far more noticeable in other cars) however, late at night I'm always faced with the dilemma of ignoring the speed limit there and bringing it up to about 100km/h. You feel nothing at all and the car literally feels like it's gliding over all the mess.
As for unsealed conditions, I don't really have much experience with Peugeots. However, I've done a hell of a lot of unsealed driving with old Volvos. They generally handled far better at higher speeds. From about 30-60km/h the car would vibrate so badly that your vision was blurred, but bridge that gap and cruise at 70-80km/h and the uneveness was barely noticeable. I've only bottomed out once, just out of Hill End, leaving a big dent in the back of the car - that road is now sealed.
But of course there are things you can do with a well-used Volvo 240 that you wouldn't dare try if you've just shelled out your hard-earned money on a brand new 407 that is supposed to remain shiny for the foreseeable future.
Patrick
605SVE