I've got the dinan stage II with GC camber plates at -3.0 with square 275s. My E90 does not understeer at all unless I get totally retarded when turning in. In fact, its a little too much on the oversteer side. I'm going to try -2.5 at some point in the future and see if it makes the car more neutral.
Maybe I'm used to my 330 but it feels like I'm always working the keep the back end in the back.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red