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.

The 8TC bi-wires are nice and do clean up some of the sound. However, at the retail price it is painful...
I sold my 8's, kept my single wire 4TC's and spent the couple hundred difference on music.

You might even try a double-bi-wire set of 4TC. For some reason, 4TC sounds completely different (WAY better) than 8TC in my set-up.

In case you want to know, I'm using a NAD 304 integrated and Triangle Zerius 202 speakers.

...And a single-run of 4TC (bi-wiring, while necessary with my B&Ws, doesn't seem to make a difference with the Triangles).

Good luck!