Most of the BMW's I see around here have higher profile "stock" tires than the "stock" tires my Crossfire came with. That's what I was talking about.
Just going from a series 35 to a series 45 you would probably feel a difference. But obviously it wasn't the tires on your Bimmer since they're as low or lower profile than what came from the factory on your XF.
If you have an NA Crossfire, that's the softest springs you're going to get. The SRT's have stiffer springs. Maybe better shocks will help a little, but I doubt if it's going to make much, (if any) difference.
Most aftermarket spring are usually stiffer, because there really isn't that much demand for "softer" springs for sports cars in general. But maybe there is a compatible Mercedes "family" sedan spring that may interchange with the springs that are currently used on our cars, and that may be something for you to check out.
The reason I suggested a higher profile tire, is because the taller sidewall on a high profile tire will absorb more of the shock you get from road's irregularities, than what the lower profile tires will.
If your roads are plagued with pot holes, there is a very good chance you will soon discover cracks in your "stock" wheels.
Stiff suspension, short wheel base, and rotten roads do not add up to a pleasant driving experience, that's for sure.
Good Luck Jonas