So here are my thoughts.....
1) The car is better with Adaptive suspension and Active Steering (I think thats right) and the magazine reviews (What Car, Autocar etc) are bang on here. It is still not super smooth on the 19's, but it removes the jarring ride from the car and steers better. If you are considering an F10 you would be missing out on the best dynamics without this fitted.
2) The 535D feels better low down, it pulls sharper and more eagerly from low revs but as I have said previously, it doesn't feel that much quicker one you get into the main torque area and feels much the same over 3K. Again it is quicker but not sure the extra money would be worth it if new, the 530D is a truly superb engine for the car, more than enough by any rational measure.
I am not sure, certainly based on where the deal sits now I would swap the car out, it isn't worth close to £9K to do so, the car is better but not £9K better so I shall most likely stick with it for now.
Oh yea, just drove the B8 RS4......review to come later.