A standard bolt regardless of hardness will not stand up to the use. You gain nothing.
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I don't follow you. Are you saying the original part is made of some special titanium-like substance that can't be duplicated by even a (metric) class 10.9 or 12.9 bolt? People have replaced that special part with an appropriate assembly of bolt and nuts.
If you look at the OEM part and compare it to a regular bolt, it's very easy to see the cross sectional difference is considerable and probable in the area of two or more times greater. Which will translate into greater strength and able to withstand the rigors of constant use and strain.
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