Whatever the merits of the various arguments, we need to dispel a myth about "European Autobahns".
Max speed limit rules vary across Europe (even if we confine ourselves to the European Union, now 25 countries), but the legal max tends to be 130 km/h. In Britain it is 70 mph (112 km/h).
In Germany there is, indeed, no legal maximum, but there is a government-recommended limit of 130 km/h. I have this notion that if you are in an accident over that speed the insurance company takes that into account, but I am unsure of that. Furthermore, many stretches of German motorway have specific limits, such as
120 km/h. And even on unrestricted stretches it is often difficult to exceed, say, 80/90 mph for any length of time because of SWOT (sheer weight of traffic).
That is not to say that people don't drive high speeds. I have been at 120 mph on quite a few occasions in Germany and Belgium (where speed enforcement is famously patchy, especially on the southern motorway through Wallonia), but I wouldn't risk it anywhere else.
Your friend's sustained 125 mph over several hours is an extremely rare occurrence.
I think that for most modern cars [even American ones.. :-) ] with engine sizes of 1.6 litres and up, cruising at 90 mph isn't stressful (in the right gear, of course). Even for 1.4 litres this should be quite manageable over the longer term.
DAS