I would add to the previous comments that I'm not sure if the Signature 2's are shielded. Judging by the photos and description provided at the XLO site for the Signature 3's I don't think the Signature 3's are shielded, which prompts me to raise that point. Lack of shielding could result in noise issues in a phono cable application that would not occur in a line-level application. There's no harm in giving them a try, though.
Optimizing capacitance is particularly important if the cartridge is a moving magnet type, for which manufacturers usually specify a recommended range of total load capacitance ("total" being the total of the capacitances of the cable, the wiring in the turntable and tonearm, and the input capacitance of the phono stage). For low output moving coil cartridges it is less of an issue, but preferably should be kept as low as possible.
Ordinary copper should be fine for the grounding wire.
Regards,
-- Al