It seems most of the modern Roof Boxes state 130kph (80mph) as the maximum speed, this is the manufactures safe operating procedure, so I would imagine a little faster on European roads wouldn't hurt, but if it all goes Pete Tong, don't say the manufactures didn't warn you.
I try to not go above 75mph in Europe, but found my speed creeping to 90mph when rushing to catch the ferry after 2 and a halve hours of traffic jams, I did quickly put this in check upon noticing, still missed ferry though, gave us an extra couple of hours of holiday though.
Do bear in mind that the roof loading figures are given for an evenly distributed load, so if the box is fitted over to one side, you can halve the allowed load.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.