Things get real complicated with suspension set-ups. There is a textbook answer, then there is the "art". One will get you close, the other takes a skilled driver and engineering team to figure out. Moral of the story -- don't touch your car's suspension until you figure out how you drive and where your car is lacking. Drive the piss out of it during HPDE's/Auto-X and have instructors relay their impressions of your car.
Quick answer is there is no quick answer.
The textbook answer is that a stiffer rear bar will provide less load transfer and make the rear stiffer, thereby producing more oversteer
BMW's have a MacPherson strut front suspension type, and the rear is a multi-link. This means many things, but in terms of this conversation it means that the inside rear gains negative camber in a turn, while the inside front loses negative camber. While cornering, this means that an already understeer prone car loses front end grip, and gains rear end grip. Thus exacerbating understeer the harder you corner. So, in an effort to maintain camber up front alot of track going e4x's and e8x's are running fairly large front bars and huge amounts of negative camber (-3.5'+). This maintains the contact patch of the front wheels and prevents rollover. More front grip = less understeer. This is why people rave that adding huge amounts of negative front camber drastically changes the handling of their BMW. Some guys all out delete their rear bar. It is backwards, but it works.
Things are further complicated when you take into account the fact that x-drive can direct up to 100% of torque to either axle depending on traction:
So, for our cars, it would be best to treat them as primarily RWD platforms. That means that an xi, would be best suited to higher spring rates all around with the front natural frequency being 5-10% higher than the rear. Additionally, camber plates would be a must. Especially if you plan on tracking the car. You can go pretty far with just that. After that, run a stiffer bar to fine tune things. An adjustable blade type rear bar would be ideal, but there is nothing on the market. If you really want to get fancy, hit up Velocity Motorcars and get their full rear link kit to make sure you are maintaining your geometery. We are still at a standstill for what to do for the front arms/bushings -- hint, hint...anyone?
In the end, it is best to trust teams that actively track their products and platforms. Since HPA seems to be the most vocal about their setups and what works, I would trust them. If you feel like doing some thinking for yourself, dig around. There is a lot of information here, but the noise to signal ratio is pretty high.
These are good primers for suspension stuff: