I've run Husky 94 (the only 94 in MB as far as I know). I've also run Co-op 91. The butt dyno tells me that shell 91 is the best though, it seems to bring the engine alive more. This is both in my M4 and my X4 M40i (pretty much same engine as M2).
I've put 89 in the X4 once (when I was considering trading it in) and I did notice a huge drop in performance and on the over-runs it didn't pop and crackle as much.
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