While a heavier gauge in a single conductor may allow a higher capacity, I am curious how signal response across the contemplated frequency range through such increased conductor cross section would be impacted. Perhaps a multiple optimum conductor scheme would be a more desirable arrangement under such circumstances, or over longer distances where such frequency distortions are compounded.
I think the trick is to design a cable that does not really favor any particular frequency. It is easy said than done though. You don't want to for example emphasize high frequency but not at low frequency - you then end up with a cable that sound bright and edgy. And you don't want a cable that has resonance issue at some specific frequencies either(I suspect some low end Monster cables suffer from this condition).
As far as having gauge size of 18AWG as optimal, that's an interesting observation. 18AWG itself may be too small to drive a speaker as it may not be able to carry a lot of current without experiencing a lot of phase shift. One way is to bunch together a lot of 18AWG wire then run them on a dielectric substrate so you can have the best of both world - low distortion of 18AWG but able to carry a lot of current. A lot of cables use this strategy especially at the high end spectrum. I looked at StraightWire website and it seems like the as the cable cost gets higher, the construction becomes more complicated in term of number of wire strands and dielectric arrangement.
It's interesting that high end cables is only a recent phenomenon. I was not around back then but in the 70's or 80's it was not an area where people were paying that much attention. Personally cables in my system have made the most prominent differences given their modest cost compared overall to my system.
Anyway, cables is an interest area of mine but as I said I don't mean to discuss too much on cables as this is mainly a Thiel thread.