Paired with a set of eibach pros is the plan.
I did have a b12 pro kit on my e87, but IMO it improved the crashyness but was a bit too firm for the crap UK roads.
So I think the B8 shocks that it comes with would be too hard for an e91.
So I would imagine that the b4s would be the middle ground? and with a bit of luck a bit cheaper as well.