Making a decision on springs vs coilovers is a common question.
Pairing a stock shock with any spring will cause it to fail prematurely over time.
Sure, you will achieve the look of a lowered car. However, because aftermarket spring rates aren't made specifically for your stock shocks, they will blow over time and use.
My personal recommendation is to do it right the first time and get the right coilover system for your ///M3!!