My speakers are also low’ish efficiency that I drive with big power amps so my HT speaker cables are bi-wired Pro9’s with two full runs/side or 6awg to each speaker.
How did your friend’s cables sound? I suspect pretty good. I have long suspected the advantage of stuff like OCC copper for speaker cables is virtually non-existent so the copper in CAT8 should be plenty good. The Monoprice CAT8 source uses polyethylene insulation, which is also good but maybe topped by a foamed insulation product like in the HT cables, although I doubt that benefit is clearly audible either.