I can second the impressions on the subframe bushings. I had them installed together with the LSD. The M3 bushings definitely tighten up the rear end, making it more precise and less floaty.
But since I'm on 19" RFTs, it's a bit on the limit in terms of stiffness. You'll notice road imperfections ALOT more. Of course this is a subjective appreciation, but I really look forward the get the Bilstein PSS10 coilovers installed which should improve the handling and the ride quality as well.
Given the rather high shop cost to install the bushings, it's best to get this done together with other parts, such as an LSD or the rear sway bar. This allows to save on labour costs.
On a daily driver, I would suggest to get a suspension first (a good set of coilovers offers the best all-in-one package IMO). After that I would look into the front M3 conversion to get rid of understeer some more. The need for further enhancement depends on the use (tracking?) and your general expectations in the handling of the car (sports car like/good handling but more comfort oriented).
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