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The factory suspension has incredibly soft springs but this is only part of the story. They are very soft and designed for runflat tyres and do not control the body much at all. They are used in conjunction with the bump stops to produce a very progressive spring rate (ie very soft and gets harder as it's compressed) infact the bump stops are engaged even at ride height and basically is always in use. They pair these with pretty cheap twin tube shocks and they wear out really quickly. They rely more on the bump stops to control the bumps rather than using the shocks.
In contrast, if you double the spring rates and use a quality damper, you can use the shocks to control the damping and the bump stop is only used to control the max compression on massive bumps. This results in a setup that can be as comfortable as stock but control the body and weight transfer a lot better.
I have driven many coilover setups on this platform and have owned Bilstein B12 and KW V2 and now KW competition. Out of all of them, the KW competition is the stiffest and the most comfortable out of all of them. That's not saying you should get racing dampers, its just a testament to good dampers make all the difference.
Heres some of the suspensions I've fitted to cars:
The Bilstein B12s were terrible. The front springs sagged and kept getting lower and lower over time. The eibach spring rates were barely any stiffer than stock but lowered the car massively, so it would scrape over everything, and compress so much over speed bumps. It was riding so low that any small bump would bottom out the suspension onto the bump stops so it was very uncomfortable. There's a lot of variants in the E8x/E9x platform to different engines and different corner weights. If you look at all the springs in realoem they are all different for each variant. The ride heights for the B12 kit are all over the place, only the E90 variant seems to have it right, all the other variants seem to slam the front of the car. Better off just getting coilovers so you can set the height to where you want it rather than hoping it'll be good and finding out you have no way of raising the car.
Koni Yellows - These shocks are very comfortable and appear to soak up the small bumps really well. They work with a large range of spring rates and still retain the ability to soak up small bumps even with much stiffer rates. Although if you do put coilover springs on will probably find the compression damping is not enough once you start driving hard and hit the bigger bumps. These shocks work really well with the stock springs and they're a nice comfortable setup.
The KW V2s I had on my car, and were a massive step up from the B12s. The spring rates are much stiffer, but comfort wise they were much better since they were riding a lot higher and weren't sitting on the bump stops. The adjustable damping aspect of them is mostly a gimmick, i have never found single adjustable rebound shocks to be good in dialing in the car in or fixing the shocks from feeling underdamped. Ultimately these performed quite well and would satisfy most people, and I suspect KW V1 would be quite similar in performance. They were just a bit too soft for my tastes as I wanted stiffer rates to control the body roll and make the car more responsive. The adjustment ranges for the height are excellent and you can run the front end quite high if you wanted to. The ST suspension all feels very similar to the KW V1 and V2s (ST X, XT and XTAs) so theyre a good option if you want the same thing for less money.
Bilstein B14 - Have fitted a bunch of these to E9x cars and they perform quite well, the monotube dampers feel a bit stiffer than the V2s, but have similar spring rates. The front springs are a bit short for my liking, on max front height, the front end is still quite low (although adding camber plates and spacer can help)
KW V3 - These feel like a much bigger step up from V1 and V2. The Compression and rebound adjustment is useful in dialling in the damping and the dampers are valved for a lot more compression over V2. The spring rates are still the same as V1 and V2, so they are still a bit soft, but the dampers at least control the body a lot better.
MCA street ultimate- These run proper spring rates for hard street and some track use, and you can spec them to have any springs you want. These feel like they are valved a lot better than most aftermarket shocks and have much better compression. These are great out of the box coilovers. The single damping adjustment I found theres really only 1 sweet spot imo. Increasing it just makes the car more uncomfortable and pogos over bumps (this is the same with pretty much all single adjust shocks ive found). Out of the box these are probably the best bang per buck on this platform for serious driving.
KW competition 2 way - These are what i'm running on my car, they were off a 130i that was racing in the improved production series. I modified the shock mountings to suit street use and fitted softer springs. These dampers are a massive step up from the consumer stuff and the dampers really work well in keeping the car settled over bumps, and getting the energy dissipated as quickly as possible. Can hit massive bumps mid corner and the car will not unload and feel unsettled, very happy with these.
Ohlins R&T - These coilovers come with a good front spring rate but a terrible rear spring rate (on non M) which is too soft. Most people just replace the rear springs with stiffer units. Because the rear spring rate is so much heavier (i think around 70% stiffer) the rear is quite underdamped on large bumps, but since most people have not experienced anything better, they just assume thats as good as it gets. The dampers do work well but they do feel a bit underdamped, even on the M3/1M variant.
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