Nice shots, Mark - and cute cats
The high ISO stuff is addicting on the 6D. As you said, there's noise, but it's clean and manageable unless you pixel peep or crop really small. I think the 6D ISO 12800 shots are good for 8x10 prints that would still look good, and the ISO 25600 would be good for 5x7 prints, maybe even slightly larger.
Here's one from last night with the 6D and 40mm Pancake, used the lens cap to prop the camera up on the curb for this long exposure (4 seconds):
Nightlife Long Exposure
, on Flickr