The phase wires can't be upgraded. The shaft is too thin to drill a bigger hole. The 4mm² cellman uses fill the hole completely already. No room for improvement here, but you can cut the near the axle and solder thicker wires there.
Sure the faster winds have a handycap here. Still, the 4mm² wires are enough to take the current of 120A of a 6T wind (short term). They are still thicker than the actual windings in the motor and better cooled, so they wont fail before the windings burn.
The resistance of the 4mm² (AWG 11) phase wires is 4mOhm for a 1m cable. So for all 3 phase wires it is a total of 12mOhm. Not so much compared to winding resistance.
The 120A cannot be used continous:
For the 6T@ 120A it is 172.8 Watts of loss in the Phase wires for the 6T, not possible to use this continuously
for the 7T@103AA it is 127.3 Watts, not possible to use this continuously
for the 12T@60A it is only 43.2 Watts, this is continuously acceptable, still the motor windings wont take it continuously
A 4T or 5T would indeed be problematic even with short bursts.
So for the 6T, 7T fans it would be an improvement to cut the phase wires near the shaft and upgrade them to gains some efficiency. I wont do it. The improvement does not justify the investment IMO