any benefit in a double bi-wire of 8TC?

I tried that once and it was too much bass emphasis at the expense of upper midrange and highs. Really changed the perspective of the presentation for the worst - a single run of 8TC was much better. Now if you want to get creative go to audio asylum and search 8TC - they have some interesting ideas...

The only way to know for certain, is to try it on your system/listen for yourself. In theory- It will provide cleaner(more faithful to the recorded material), improved sound, which of course: some don't like(not their taste). Anytime bass is increased relative to the rest of the audio spectrum, the "rest" will SEEM attenuated. The up-side of purchasing used Kimber products is that, should you not like the effect; they can be resold at little(or no) loss and usually- quickly. Keep the runs the same length(hi/lo, same channel), to avoid any imbalances.

Talking with Kimber Kable they told me the best sonically sounding version 8TC speaker cable is the internal bi-wire version. They got into technical detail and says the coherent signal path is created equal which perfectly matches the impedence.