A Tale of Many Sway Bars
One of the more complicated things I've done is swapping sway bars multiple times on my vehicle. The swaps aren't simple due to needing to drop the subframe each time you do it, for either the front or rear you're doing it on.
My F31 328dx came with a 21.8/23.6mm front bar and 13mm rear bar stock. The first bar I changed was the rear one (because it was easier than doing the front bar) and installed a KC design 17mm solid rear sway bar. At the time I had 200lbs/in front springs, 650lbs/in rear springs, and FCM Stage 2 tuned dampers. This combo felt fun in daily driving and good on the track. I could've used some more front roll stiffness though.
Rear Sway Bar Install Video
So I began researching front sway bars, and for xDrive there really aren't many options. For standalone individual front bars you can buy there's only the 6cyl BMW OE one (which wouldn't have done anything for me since it's the same thickness), UUC Motorwerk's, and H&R.
UUC Motorwerk's size/thickness of their bar looked pretty good from my calculations, but that company is a complete dumpster fire. They're absolutely terrible to work with and ALWAYS miss their commitments. Even worse though, when I finally did get their bar, it was very obvious that they don't actually have an xDrive compatible front bar even though they advertise it as such.
So I ended up ordering the H&R bar. Now H&R doesn't "officially" sell just the front bar standalone, but some of their vendors will, so you can get it that way. This bar is a ridiculously beefy 28mm solid bar. I could immediately feel the added roll stiffness after installing it. That said, this bar was way too stiff for my setup. In daily driving it took away too much suspension independence and made the ride quality a lot worse. On the track it was obvious it was too stiff (even on it's softest setting) as the car didn't want to rotate on tight corners and produced too much understeer. I wouldn't recommend these bars for spring rates much above stock. If the H&R bar was too stiff with 200lbs/in front springs, it would be even worse when I went to my 240lbs/in springs for my FCM Stage 3 config.
Front Sway Bar Install Video
If you're not looking for standalone sway bars, then Dinan also makes a set. They're by far the most expensive, but I knew their front bar was the size I needed. Given that there weren't any other options, I paid the Dinan tax and bought the set. After getting the Dinan front bar in there the car felt so much better balanced! Sure, it had more roll than with the H&R bar, but the car rotated better and as a whole felt more like one solid mass instead of distinct front/rear halves.
FCM Stage 3, and Moving to a Smaller Rear Bar
After getting my FCM stage 3 setup (Front 240lbs/in, rear 900lbs/in) and hitting the track I soon realized that the 17mm KC Design rear sway bar was too stiff for my new rear springs. It was way too easy for me to start getting the rear end out and I had to be ULTRA smooth with my pedal inputs. To make matters worse it rained heavily the night after the first day and the second day the track was really wet. To give myself more margin I disconnected my rear sway bar
which ended up being the right and wrong thing to do. That experience told me I absolutely needed to find a smaller rear bar though.
The next smaller standalone bar from the 17mm one I had is a BMW OE bar for the 2 series that's 15mm solid (PN 33536853476). BMW of SF had them for just under $150, so I grabbed one and swapped it in. For daily driving I didn't notice the KC Design bar being "too much" other than on some fun roads where the traction control light would flicker, so I'll have to wait to get to the track to really see how well the 15mm bar does. That said, I don't notice the TC light flickering as much on fast roads with lots of bumps. As an added benefit, the weaker coupling between the sides of the car has improved ride quality a bit.