There are 3 common positions for the M30 in the E30, I'm sure it's about the same for the E21. If it's in the farthest back position, I don't think that it affects the weight distribution/balance very much. Obviously it will be heavier, but the position may mitigate some of the weight issue.
Like I said, this has been pretty well documented for E30's, I'm sure that E21's can't be very far off.
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