As far as I know, CGX -always- use CHIP memory for processing images.
Mmmh, CGX keeps planar image in chip ram for legacy compatibility. But real gfx data is in a VRAM or fast RAM.
One of the reasons why they don't recommend opening any AGA windows. Unless someone can prove me wrong.
Errr.... You must be confused. There are no AGA "windows". You can have AGA
screens with CGX, there is no problem what so ever.
Please also notice that software can allocate chip ram bitmaps even if RTG system is in use. Amiga defaults to planar gfx and it must be in chip ram...
EDIT: Of course if you open AGA screen you lose some chip ram because AGA screens must reside in chip ram. That is how it works.