Ideally you want a gauge with say a max of 2 or 3 bar range as the difference between .8 bar and 1 bar boost could explain your problem.
Having said that I am at .5 bar at 110 kph on a flat road.
An egt gauge would also help greatly in this logic maze
In the absence of that maybe do a run on your motorway at night with a buddy driving behind you as then it will be easy to see if your a smoking at the 60 max that you getting - in which case fuel delivery isn't the problem
Also try and see if while sitting at this 60 max whether a couple of blips on the throttle doesn't cure it even for a couple of hundred meters at a time. Not sure what road in problem solving that might lead to though