I would strongly advise against just changing springs as well. The suspension is a system of parts working together .... the shocks need to be paired with the correct springs otherwise you throw it out of balance. I cringe at all the cars on this board 'dropped' on just springs.
Also read up on the XI suspension, it is not the same as the standard or sport suspensions on the RWD cars. Especially when it comes to upgrades.
2011 BMW 335i E92 - Le Mans Blue, 6MT, M Sport. JRZ RS Pro, Quaife LSD, BMW Perf Exhaust