Some Naim purists insist that using Naim's own NAC A5 cable is the only way to go. This was true for older Naim amps, but the Nait 5si is not in that category.
I've owned the 5si and the XS2 quite recently and have not used Naim's speaker cable on either.
The older amps are engineered in such a way that the NAC A5 was an integral component to the functioning of the amps (I'm no engineers so don't totally understand the principles involved, but this info is out there on the Web if you care to look deeper in to it.
The later amps, such as the 5si, XS series, and Supernaits do not have this limitation.