I would think that CDS / Cold Drawn Seemless tube was specified to stop people using any old length of tube and not knowing what it was for the wishbones. ERW will be fine for the wishbones although I would not go any thinner than 2mm wall top and bottom,don't forget that they are only bolted to 3mm suspension brackets and half of those have 50% material coverage welded to a 1.6mm walled chassis.
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