> OS4 being one step ahead providing Polaris support

Depends on how you count "steps". If 1 step = 1 GPU microarchitecture generation, then we have in terms of support in MorphOS and OS4:

1. R300-R500: MorphOS (2D+3D), OS4 (2D)
2. TeraScale 1: MorphOS (2D), OS4 (2D)
3. TeraScale 2: MorphOS (2D), OS4 (2D)
4. TeraScale 3: OS4 (2D), *
5. GCN1: OS4 (2D+3D)
6. GCN2: -
7. GCN3: -
8. GCN4: OS4 (2D+3D)

In these terms, OS4 is 5 steps ahead in 2D and even 7 steps ahead in 3D :-)

* "MorphOS will boot, but the display is very flakey."

> you won't miss much if you use at least R600 Radeons. [...] 3D acceleration works

...except if you use MorphOS ;-)

> What benefits you do get from >=Polaris cards?

Better availability of non-used cards, I think.