[caravela]

For 512mb boards which is likely to be the most optimal setup, a GPU/ARM split of 128/384 or 256/256?

that depends on what you use your pi for, if you just run Xbmc 256/256 or 128/384 should be fine. if you are gonna have services running like uPnP, webserver, transmission, samba i would recommend the 128/384. Again just depend how much stuff you have running, more stuff more ram for the system

EDIT:just tried 128, 192 and 256 by bare observation i couldn't see any difference

Of course the memory allocation is now much more granular, in 1MB increments, so splitting the ram in 128MB chunks is no longer necessary. You could allocate (on a 512MB board) 192MB to the GPU, or 208MB, or whatever amount works best for XBMC. Heck, even on a 256MB model, you could give the GPU 144MB (an extra 16MB) if it makes any difference.

Yes, that is why we only have on self to choose from