ZII is alwasy slower then AGA bandwith wise that's just a simple fact.
The point where ZII GFX cards score is when resolution and color depth is increasing.
While the bandwith for ZII is lower then AGA the GFX chips itself are much faster then AGA. So when resolution goes over standard PAL or NTSC the bottleneck is not the bus bandwith but the speed of the GFX chip.
While PAL highres 16 color is unbeatable by any ZII GFX card the equation looks different when it comes to PalSuperhighresInterlace 256 colors, AGA is just bog slow in rendering such a "huge" screen while any old CL5426 is having a good laugh at it.
