Bilsteins on your M3 all depends. On my convertible it is just right. It is NOT too stiff. If I put these on my sedan it would beat the hell out of it. That is why I use Koni on the sedan or the Sachs/Boge OEM. Coupes and sedans ride quality will be ruined by this stiff a strut.
Also in regard to tightening the top nut on the strut unless you have the small wrench that holds the shaft it will spin on you. Also if the shock is fully entended where any of the shaft is showing (most likely requiring not installing the lower bolt until after you do this) you can get a strap wrench on the shaft which will allow you to hold the shaft and tighten the nut while not damaging the shaft. This has worked well on almost all cars I have worked on unless they have a full shroud that protects the shock...
Bentley page 310-4 shows two torque values for the strut nut one for shaft with external hexagon and one for internal hexagon 48 and 32 ft -lbs respectively so my guess of 22 is wrong. I don't know what a internal hexagon is which I am guessing may be an area where you can get an Allen wrench on?