Yeah, m3 front arms won't be the culprit. They change the camber, but you can't really adjust the resulting camber much. Standard toe setting with m3 arms will work fine, though 0 toe is probably better overall (or negative toe for track use).
The shimmy when hitting a bump mid corner is exactly the symptom of sloppy rear subframe bushes, so I'm told. Mine does it too, so they're top of my list.
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