One of the problems I find with high speeds (posted or not) is the differentials, particularly where there is significant truck volume. I find this most noticeable in 4-lane sections of Hwy 401. Trucks loose momentum on long grades, other trucks trying - very slowly - to overtake them, which jams up everything. Even in 6-lane areas, this leaves Lane 1 for the higher speed traffic, but the real flyers get jammed up by those only willing to do closer to the limit.
Do European highways have similar truck volumes?
Once tourist season comes, most motor homes and vehicles towing trailers won't be taking advantage of the higher limits.
Maybe if we had better driver training, cars and transports, things would go better. Some people simply don't have either the skills or confidence to be at those speeds.