For what it's worth my RA65 with modded 1G, good tyres (Falken 452 in very similar sizes to you - 225/45/16 & 205/50/16) and LSD is FAR better in the wet than it was with the 22RE, open-diff, worn 14s, and otherwise same suspension - and A6s are known for being quite light in the rear end.
I would have thought Z2 would be better in this regard - so so it really sounds like tyre or maybe suspension issues.
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