ajsalida is right about the XI suspension, it's horrible.
I've spent years on track, first racing GP motorcycles in both Europe and the US and then Karts, and cars, up to instructing in high horsepower AWD Audis before I hung up my track shoes and nomex a few years back.
First, the car needs a rear bar FAR MORE than a front bar on track. The cars are way to tight and literally plow into corners. More front bar just makes it worse.
An Xi will pick up an appreciable amount of camber as soon as you lower it to a reasonable height, even without adjustable CA's, and that will improve turn in a bit (as will 0/0 toe on front) but the last thing these cars need is more front bar until your spring rates are WELL north of 600. Trust me, you're not going there on a car you use on the street.
The Bilsteins are okay, as are the FSD's if they are properly sprung. Both benefit from the Eibach pro springs. Beyond that look at Eibach's ERS springs, they are better than any of the other common tuner springs most people go with (and I've seen the H&R's fail). Check vendors in Germany and the UK for the correct springs. Eibach's distributor in the UK knows their stuff.
A better setup altogether would be the Ohlins road and track setup. It's as good as you're going to get without going to TTX36's or a 4-way Moton setup, and that's gunna be in the $7k+ range to start.
PSI at Sears Point (Sonoma) Raceway can put a set together for you for just around $2000 and that may be the very best suspension shop you'll ever work with unless you have a truly big dollar racing program (and even those guys use PSI). They have done suspensions on two of my track cars and I have never found anyone close to them in the civilian market.
The OE bushings, in particular the fluid filled thrust arm bushings, are a complete train wreck. Get a press and swap them for high durometer poly. Do the same for the rear upper bushings. If you can afford it, do every bushing in the car. Just the thrust arm bushings will change your perception of the car's handling. Do all the bushings and you'll think the car can read your mind compared to the OE rubber.
Do not skimp on the bushings. The difference is NIGHT AND DAY.
As for swaybars; most people put too much bar in their track vehicles. Get the spring rates sorted first, then worry about swaybars.
When we did setup in my 440WHP (520+ crank) 3100lb Audi we did initial rate setup on the suspension WITHOUT ANY BAR. Once we had it dialed we went to a blade adjustable rear bar and a slightly bigger front bar, only to go back to the OE front bar. The car was ultimately a little loose, but very, very fast. The BMW has proven to be very similar in that respect (albeit better out of the box, but not by much).
Cars with too much bar tend to exhibit very twitchy, light switch handling characteristics on the edge. A properly damped and sprung car with less bar will be much more predictable and easy to drive at speed.
So, my advice..
1. Bushings throughout
2. Don't even bother with the Bilsteins or Koni's and go right to the
Ohlins with Eibach ERS springs
3. Proper wheels with the correct offset so you can maximize the car's footprint width. Spacers if you can't replace your wheels. It will make a noticeable difference
4. Rear bar (though if you're doing the rear subframe bushings you may as well do this then as you're already there)
Then go drive the hell out of the car and destroy a few sets of tires. You'll end up getting the car dialed in for your favorite track and learning what you want in terms of tight/loose playing with the bar. Guaranteed it'll be different road vs track, so make sure that rear bar is adjustable. I actually run more bar on the street than the track, believe it or not.
The UUC bars are good, not many others are. The UUC bar on soft will get the car to rotate for a change and then you can dial the cars compression and rebound to your liking and getting the alignment/camber so that your tires are heating evenly. Aim for <8°F difference across the width of the tire as a starting point. Adjust camber & temps till you find a sweet spot. This will all change as you get faster.
Once that's done start playing with more bar.
If you're not on full hard with the rear bar, adding a front bar will be pointless. If there's too much roll, try by adding compression damping and/or higher rate springs. A bar is no solution if the dampers and springs aren't dialed. It only masks the real issue and slows you down.
After you get the car initially set up, take notes, lots and lots of notes, and use the expertise of the guys at PSI, they know their shit and work with some of the best teams in the country on their setups. This is the main reason for buying from them.
Your next problem is going to be brakes. Once you start modding the brakes be sure you don't make the same mistake most make and screw up the piston ratio front:rear. Once you alter that the EBD is going to be totally messed up. Without reprogramming it and the car will be a disaster on corner entry, especially when trail braking. FYI - I don't know anyone who's successfully reprogrammed the brakeforce distribution on Bosch ABS controllers without shelling out huge $$$ for parameter definitions. Remember it's all tied into the traction and stability management. It's really easy to mess the car up here.