@Brian - as mentioned before on here, I hit 165mph indicated (true 150mph) in Germany and except for the extreme mirror wobble the car felt pretty stable - and the brakes pulled the car up strongly without it feeling unstable. The car feels "ruffled" at those speeds though and far from the contented 100mph cruiser it can be with the roof on. (Which reminds me I need to get silicon spray for my roof rubbers as they were squealing like a piggy this morning.)
I have hit an indicated (terminal) 270kmh in an Alpina Roadster S and 270kmh (indicated) in the B3 Coupe - both felt far more attached to the road (Alpina body kits are specifically designed to remove lift) with the B3 still pulling - just not particularly hard.
Back to the original point - and my drive in this morning. The SS definitely prefers good tarmac - and imperfections at slow speeds can make it feel fidgety but not disconnected. As mentioned already, maybe a tweak to the suspension settings might dial back your GTN to a more acceptable level
