Hiya, I've been on the A1 (dual carriageway, runs almost the length of the country) with a hefty laptop in it at 70-80 MPH (indicated) and it's fine. I don't go any faster. I have the 25L Krauser panniers and they have a similar effect i.e. you know they are there (just) at motorway speed in amongst the lorries and side winds, but nothing near terrible with either. I have not tried the ensemble yet.
It is quite a modest size top case (marketed for motorbikes or scooters) and egg-shaped (aerodynamically better than a box), so maybe that makes the difference. I do have fresh suspension, bearings, modern tyres and frame braces. I'm also 1.9M tall and 140kg. I guess that might stabilise anything, in the way of a bag of cement
On the B roads (50 and 60MPH) I have to check my shadow on the road to see if the topbox is still there!
I do like the RT fairing, cage drivers definitely notice me, flying pests generally get deflected and it is a rain-shelter as long as you keep moving. I also like the look. It's all personal taste though.
Hey, what do the 'period correct BMW Touring cases' look like and how do they attach? Did you keep the Krauser lower frames?