It all depends on your car really... some cars are designed for those speeds so can actually be more efficient at those speeds because of the RPM required to get to those speeds.
I also think that the majority of cars using the roads in question are capable of those speeds without danger of sucking the oil reserves dry.
Another thing that increasing both the speed limit and minimum speed would do is help traffic flow better reducing congestion. At the moment there is too big a gap between cars driving on the roads so it makes it more difficult for traffic officers to predict flow of traffic and therefore make it harder for them control the flow.
The biggest problem is the lack of a minimum speed limit tbh so if they don't like the idea of raising the maximum limit I would certainly like to see them introduce a minimum as the minimum limit is more important.