IMO, the stock suspension is pretty good. I don't think any spring or sleeve kit will be a significant upgrade. I have the dinan stage 2 w/GC camber plates. The stage 2 didn't do much, the camber plates really helped with front end grip and tire wear.
I think the best thing you can do keeping the OEM shocks is front camber plates, solid rear subframe bushings and bigger sways.
The old E90 ring taxi's were very fast...all OEM to include the PS2 tires. It did have upgraded brake pads. Keep in mind that it almost always had 4 people in it.
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