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The Pro kit also comes with a modified lower ball joint mount that corrects the caster issue when lowering these cars that the standard kit doesnt.
what he said ^ ^ ^ it basically changes the mounting point of the wheel so before the additional features of the pro kit the wheel would mount like this:
after the pro kit it would mount more centered in the wheel well like this:
this really only becomes an issue when you want to run supper low on some big wheels.