With the 260, in the long exposure noise mode, iso's start at 140, then 200, 400, 800 (I am not sure about above 800 i.e. 1600 and 3200).
I am not sure if sensor plus works in the long exposure mode or not. Sensor plus at 400 is very good albeit 1/4 of the original file.
From reading the post, the image posted is a 30 second shot at iso 50, which is using standard iso settings, but the file is very clean for sure.
I am wondering where the 260 starts to show excessive noise in normal mode at iso 50. 30 seconds is pretty much the max on the 160 and even at 50 is pretty harsh with noise (again depending on outside temps).
On the post's shot of the skyline at 30 seconds it's still very clean with very little destructive noise showing.