90 or 100 seems safe enough to me, in the right conditions, if there aren't many other cars about, so cars doing 70 in lane 1 could be passed by cars doing 100 in lane 3, intentionally leaving a 1 lane gap.
I don't see much point in cars doing any more than this, and I bet the risk curve goes up quickly as most cars don't handle great over 90, and most drivers certainly not good enough to be driving over 100.
Autobahns don't make much sense, but neither does a 70mph limit on a motorway with little amounts of traffic, in modern and safe cars.
If we're thinking about emissions though, which we should be for the next 20 years until we get a grip on it, then I'd limit every car to 80, other than cop cars. Drag massively increases with speed, fuel/ electric use goes up, as do direct emissions (or power generation emissions).