I have a Pentax K-5 but I don't shoot a lot of night scenes with it. There's a noise reduction feature in Photoshop when opening RAW files that is effective.
As far as noise goes, I find the K-5 is very good. It handles 30 sec at ISO 1600 very well, at least for deep sky, even in the heat. The K-5 also runs pretty cool. This is evidenced by the amazingly long battery life I get out of the thing. I can go all night on one charge, unlike my old K-x and Nikon D70s. The Sony A7s would certainly be better, but that's a new camera.
Now I'll have to go shoot a few wide shots with my K-5 to see how it compares.
