Andreas is, no big surprise, right - I don't have an 8610. But I always liked to have seen one emerging on the market and would have suggested Acube to base their next Sam on it. Would have been the way smarter choice IMHO.

The question whether the internal DiU is blocking the cpu or not is far from trivial. best choice is a 8610 board plus a (powerful) pci express gfx card with dedicated VRAM. But the DIU isn't as braking as the custom chips on Amiga were. The RAM controller runs at full speed (up to 533 MHz). The the maximum bandwith is shared between all DMA units attached to the bus. Since the bus is fast I woud expect a better performance with shared memory on a 8610 than with a slow bus and dedicated RAM on a 7447. Anyway, for high performance you better add a dedicated pci express gfx card and then the problem is void anyway.
If you want 3D you'll need a dedicated fgx card anyway. But on youtube there is that demo which shows that the 8610 is cacpable of hd video - I think that shows quite well, that it is actually rather capable.
Anyway - it's all academic. The 8610 is ageing. Today and tomorrow QorIQ the best route for PPC (if at all that is).