I think 90 or 100mph is too much really. 80mph is the way to go, because you have to take into account people in dinky 1.0ltr cars.
If you raise it too high then they simply cant keep up and have difficulty overtaking, not to mention their fuel economy going through the roof as the engine screams to reach 80mph. Driving too slowly on motorways is more dangerous than speeding....then there;s those people who just sit in the middle lane