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 E92 335i, 6MT, M Sport, Le Mans blue - Michelin PSS, Quaife LSD, JRZ RS Pro, JRZ camber plates, Eibach ERS, full M3 susp, custom toe arms, Powergrid end-links, brake ducting, SS brake lines, UUC SSK, ETS FMIC, ER CP, Perf. Exhaust, work by VAC Motorsports in Philly