Is it safe to conclude that although the X's are great, the 6D still beats them soundly?
Looking for a new cam and although I'm in love with Fuji, money is tight and I need the best ISO performance possible.
I wouldn't guess soundly, but probably by a little bit. Looking at DPReview's studio comparison tool, the X-E1 is actually a little cleaner at 3200, but the 6D resolves more detail (expected). Have a look and see. Note that the FUji is about 1/3 stop over rated on ISO (so ISO 3200 is actually about as sensitive as ISO 2500 on the Canon), but even with that, you'll still see they are pretty close overall, with the Fuji slightly cleaner and the Canon resolving finer detail.
The real big difference between them is going to be autofocus in low light, and there the Canon will be significantly better. Low light AF is really where the X-E1 struggles, sometimes failing to lock AF.
If money is tight, the 6D would be one of the last cameras I'd buy, to be honest. An X-E1 is a lot cheaper, lighter and more portable. They are two very different cameras. I can't see any good reason to chose the 6D over the Nikon D600 unless you've got a lot of Canon glass already either.