And another favorite video:
This is a test of the first year Z4 3.0, 225 hp. The 2006 - 2008 3.0si is 255 hp. However, this video clearly shows the spirited way you can whip around in a 3.0. The 3.0si has an extra 30 hp, for even more spirited fun. So, you don't have to buy an M to have a lot of fun.
Although you may want to!