So this whole, (what I consider crazy with my background) thing has developed where F3x guys spend all kinds of money on heavy springs, shocks, coilovers, etc pretty much in an attempt to handle body roll. Rather than just upgrade the component that most directly controls body roll. Even after all of those upgrades, most of them are still running stock sway bars!
My local BMW sales rep who used to race cars for an actual living totally agrees. He said that the very first thing he would do with an F3x is upgrade the sway bars.
H&R has great engineering. Their sways are a great heavy duty set that are perfectly matched front to rear. My car corners FLAT with them.
Based on your car, requirements and budget, then I'd go with H&R springs and Koni Yellow Shocks.
Top off the front suspension with this strut tower brace...
https://www.kiesmotorsports.com/prod...f32-f36?aff=22
The most important bushing change that you can make in the front is to go with these monoball thrust arms. It gets rid of the vague F3x steering without adding Noise Vibration or Harshness. This makes the steering very precise which is imperative on the track. Huge difference.
https://www.kiesmotorsports.com/prod...set-rwd?aff=22
If you have budget you can replace Lower Control Arms (wishbones) with monoballs also which tighten up side to side movement. But UCA are more critical for the huge steering improvement.
IMO avoid the old style factory shaped bushings made out of less compliant materials. They just add unnecessary NVH. There are monoballs for the rear too. But I'd suggest doing the above a drive the car before jumping into more rear suspension changes. You are already changing rear springs, shocks and sway bar.
I got lucky last year that BMW had a fire sale on the M Performance LSD that fit my car. An LSD is a great addition and there are a few options available to you. It's expensive so drive the car with other mods first before pulling the trigger on an LSD.
Tire size and compound make a huge difference. I stuck with a square setup on stock 400M wheels with my xDrive and went 245/40-18 on Michelin Pilot Sport 4S. Amazing! If I went with 19" wheels, I would have done 255/35-19.
I don't think you mentioned whether you have grey brakes (Front 340mm/Rear 330mm) or blue M Sport Brakes (Front 370mm/Rear345mm)?
I have suggestions with brakes for tracking too.
Hope this helps!