If you will forgive the intrusion (I was a dedicated supporter of the greater good....but I've lapsed), I really don't think Tau need to worry about GSC.
I'm planning a GSC army. I've been reading the codex for the last week or so, and theory hammering various scenarios, and the one that always stumps me is Tau with lots of early warning overrides. Unless the GSC goes first, and rolls a bunch of 6s, a Tau army will absolutly wreck it. I just cann't see a way past intercept and smart missle systems. Even if the GSC player goes first, with a few units of kroot screening your heavy hitters, it's going to be a tough game for the Cult.
As others have said, the only option the GSC player has is to play strictly to the mission and almost forget about killing the Tau army.
Against any other foe, I think the Cult will be pretty strong, but Tau will be a real struggle.