I'll preface this with I've not run SWB however I would like to know what tires you are running. I feel that tires and bodyshell make a big difference to how my Mini handles. With that I find that I run different spring setups for different tires.
Also with the M05 A rear sway bar is helpful. I found it made quite a difference.
The other thing you can look at for the M05 SWB is a large amount of rear toe-in.
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