Before you change out the stock springs, take a look at
this thread. I spent a lot of time considering these options with my E92, and generally speaking going with any kind of "drop" with springs results in unpredictable handling, loss of EDC functionality, and other issues. Dinan gets away with it by modifying the stock bump stop, which on the M3 acts as a helper spring and is actually an active part of the suspension.
There are ways to make lowering springs work, but there are things to modify and consider before changing out the springs. Here's another
thread that might be helpful too. Folks who want a sport suspension like the KWv3s, but again EDC is lost and the ride suffers IIRC. The stock suspension, while looking a bit SUVish, is damn good and hard to improve upon. There may be more out there for the E92 now, and my knowledge is based on research from a year and half ago. But FWIW I thought I would share what I know.