Generally speaking, I'd have to offer a guess of at least 2.7º, and maybe more for auto-x, 3.0º negative camber at all four corners. Front wheels toed-out a bit for improved turn-in response. If you go to the trouble and expense of modifying your suspension to get that much negative camber, generally speaking you should end up w/ pretty good grip in turns, but you'll be wearing out the inside edges of your tires in daily driving unless you have them flipped on the rims every 3k-5k miles. Might have less grip in straightline wet driving (maybe more tendency to hydroplane) as you'd be driving on the edges of the tires. The toe-out would decrease the self-centering of the steering/front suspension/wheels, so you might have to make more steering corrections in straightline driving, car might be a bit 'darty', maybe tend to tramline more easily. Suspension setting choices are always compromises between varying choices & priorities.
Wouldn't maximum straight line breaking also be decreased slightly? Also the stock setup only allows for a maximum of 1.5 degrees?
Apparently there is no camber adjustment on the fronts, and putting camber plates in would kick me out of stock class. Therefore I didn't do much as mostly I want it to be a good daily driver. Zeroed out the toe in and put a little more negative camber on the rears. I did put a heavier front sway bar on though which feels good. By the way I love my Dunlop Direzza Sport Z1s. I have an autocross this weekend I can't wait to see how it does with the new sway bar.
There is some camber adjustment up front similar to the E46. You need to remove the camber pins on the top of the strut towers, and then you can slide the struts all the way toward the inside to get the max negative camber available (factory has slotted the top of the strut towers). I've not done it on my E90 yet, but on my E46 (D-stock autox) with the pin out, I can get ~1.2 degrees of negative camber with me in the driver's seat. I've heard that on the E90 you can get slightly more. I've tried an aggressive autox alignment on the E46, but the toe was too radical for highway driving. I used slight toe out up front and zero toe in the rear. The car was way too darty on the highway and wanted to follow every feature of the road. I ended up using the max neg camber up front, toe-in set at the min of the factory range up front and middle of factory range in back. With me in the car, I set the rear camber at the min of the factory range (I think it was -1.5) in order to facilitate rotation. I would guess something similar would make sense on the E90. You definitely don't want more negative camber in the rear -- the car will understeer too much as is.
On my E46 we cranked out as much negative camber as possible (just under one degree in the front, and maybe one and a half in the rear), maybe an 1/8" to zero toe in in the front and 1/8" in the rear toe out. Now I have been running a touch of toe in in th erear too for better transitions in slaloms as a compensation to the no limited slip. It does work. My E36 318i still has a better handling car because it does have a limited slip.
