If you have, or can borrow, a hand held GPS unit, it may be worth doing some trial & error testing of the performance of the 2 heights (also experiment with different trim positions). Record your findings. If the present position does give the best performance (which is probably why the previous owner positioned it that way), you could always add a wood block to the top of the transom. This would allow the the outboard collar mount to apply most of the weight of the outboard to the top of the transom (via the block), and minimize the shear forces on the through transom bolts.
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