I want the same effect as with Super + W but for only the selected windows.
Is this possible to achieve this?
Yes, it's very easy. Press Super + [number] twice.
For instance, if you want to view all the Nautilus windows you have opened, press Super + 1 1 if Nautilus is the first item on your launcher.
Also, clicking on the icon of an active application that has more than one open window has the same effect.
Looking at the Unity source code, this sure is possible, but isn't easy. Unity implements such a view, using the Compiz Scale plugin. First, Unity generates the list of windows of an application and then passes this to Compiz.
So without Unity you can't do this (as for now), since such a feature isn't in Compiz out of the box.
Theoretically, yes you can.
Launch CompizConfig Settings Manager > Scale > Bindings > Initiate Window Picker for window group.
I say theoretically because it doesn't work for me (but it does work for others).