Can you tell us the length of each wire run? That is the anticipated length from the amp to the farthest speaker?
If all the distances are less than 50 feet, there really is little or no need for wire that heavy. 2.5mm² is more than enough.
The particular cable you are looking at is low capacitance, and given what little I know about the general construction, it would likely be low inductance as well. Consequently the only thing that matters is Wire Resistance which is about using nearest equivalent in AWG wire,
* 6mm² is about 2.6 ohm per KILOMETER.
* 4mm² is about 4 ohms per Kilometer.
* 2.5mm² is about 6.5 ohms per Kilometer.
That means an entire 100 meter roll of 2.5mm is only 0.65 ohms. 10 feet is 0.065 ohms. Generally, if you stay below 2% of the speaker resistance you are fine. 2% of an 8 ohm speakers is 0.16 ohms, 2% of a 6 ohm speakers is 0.12 ohms, and 2% of a 4 ohm speakers is 0.08 ohms.
If you want to spend the money, then fine, it is your money. But as I said in your other thread, unless you are connecting a car battery or 1000w/ch amps, they is NO need for 6mm and very little to no need for 4mm.
Steve/bluewizard