Same as above for mine too, runs arrow straight and completely calm at 90mph. It had the shakes at 60 to 70 mph when I bought it (the previous owner didn't go over 60 to avoid this) but sorted the problem and it's fine.Steady 90mph in a RRC??? You're quite brave sir!
It's a consideration when we take it the the Alps through France, almost entirely two lane on major route and 130kmh with many HGV on inside lane. Either you have to dodge in and out into that lane to let faster vehicles pass you or match them. Generally, flow unimpeded is about 88mph (90mph speedo and 88mph gps) so we just fit in with that.
Fortunately that's almost right at engine torque peak for my 3.9 at which it feels relaxed. I honestly thought that elevated speeds would severely take their toll on fuel consumption, but it's the most economical right there. Fuel light on when flat and level gives a repeatable 40 miles to get more, 50 and it'll be dry!