Simply load the demo "Rain" by The Black Lotus. The clouds will look crap with 16bit scandoublers like DCE/phase5 CV3D scandoubler.
In contrast Arxxon or Picasso4 will look perfect.
BTW, AGA has a 24bit palette. That means I can make smooth transitions of just one bit. And it's noticeable if you are used to painting and computer graphics. It doesn't matter if you are showing just 16colours, it won't look smooth (read right) with a 16bit scandoubler!
Regarding HAM8, you can make very smooth transitions just changing one bit. The problem is not smoothness, the problem is changing from one colour to another one completely different.
In addition to that you should remember that AGA is not limited to 256 colours. Each few pixels the copper can change the palette.
If you can't notice the difference of AGA running through a 24bit scandoubler and a 16bit go to an eye specialist because you have some kind of visual problem
