55s would be my choice, if there are suitable wheels. I'd be happy with more rubber on my car but not on Geoff's.
Geoff bought the wrong car - or the right car in the wrong configuration. His test drive was too short, or was on flattering roads. He's now looking for a softer ride, which will inevitably have an impact on ultimate handling. It's a compromise he has to make if he can't tolerate the ride quality.
I'm not sure that just swapping from run-flats will give him what he wants, so have pointed out that the more usual solution to a harsh ride is more rubber. Ultimately, I would take the advice of a BMW dealer who must know the pros and cons of all combinations.
Generally though, I think skinny tyres are a bit of an affectation on road cars, especially those that are meant to whisk you across Europe in comfort. British roads, in particular, are not smooth. Most roads have pot-holes and poor repairs.