well if thats true then owners of GTX and Ultra cards will also sol and have killed any overclocking headroom since they use similar wattage, or just a bit less than the XT. Although I don't think you would agree with that, would you.
It's a fact. The XT consumes the same or more power under load, than even the GTX and the Ultra and is considerably slower, so I fail to see how that is a XT saving grace...
Besides, the Ultra is a silicon revision of G80, so they have plenty of headroom for overclocks.