+1 no bigger front bar needed.
Maybe on E30s, E36s this was relevant.
With E90, because the lower knuckle has 2 ball joints, it means the actual pivot point of the knuckle is out in space (read - lower pivot point is farther away), yielding a more desirable camber curve - despite the thing being a strut car.
I detail out what the car needs in a blog post. I too have spent considerable time on setup (less on parts - but more on setup!) to make the xdrive work. Apart from coding the thing so that the awd system does not get in your way (read: bmw built in so much safety crap into xdrive, brakes and rear ediff that even with full traction off, the computer is doing things to intervene - this applies to rwd cars as well). The biggest thing is loosening up the car. From factory, it's setup to be very safe--at the great expense of having a car that you can work with (read: manipulate as a driver).
You can't over lower the car - even beyond 1 inch. The rear toe is not as adjustable as desired and as a result, dropping the car (even on eibach prokits) will yield rear toe-in at levels that really lock the rear axle down. Great for street, great for confidence but not great if you're trying to set the car up to freely rotate based on driver inputs.
So depending on ride height, you will need to get rear toe-links to keep that in check. Front camber plates - ideally one that is not bushing based. There's too much deflection in the front rubber shock mounts that even if you remove the alignment pins and get some camber, the front end washes out very early no matter what you do. Take that deflection out, and you will have a car with much higher grip limits on the front axle.
From an alignment perspective, that's going to boil down to how you want the car to drive...but the big things on xdrive are - keeping alignment in check, removing front end deflection and playing with rear roll stiffness.
Regarding swaybars, NinetyOne has it down 100% right. Actually, even on *stock* xdrive swaybars, I am lifting up the inside rear tire on every corner at a local circuit - this is with trail braking of course. Putting a rear bar is just going to make this happen sooner (read: rotation can begin earlier)....so consider that as well - and if that fits your driving style. Detailed musings on the topic here:
http://dreamingin302ci.blogspot.ca/2...ve-handle.html