I don't know about these things, but I do know that there is more to the stiffness than just size. IF the aftermarket bars are made of a different steel, there performance would differ even if the bars are the same diameter.
No doubt FW28z knows more about these bars than I do, and I wouldn't be suprised if the aftermarket folks don't make their bars out of larger stock, even if only to give their customer's the warm fuzzy about what they are selling.....
Regardless, for waht it's worth I'd guess it's a safe bet that a reputable company's aftermarket sway bar will out perform the stock even it were the same size.... which apearently they are not anyway, so mute point, eh?....
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