If the die size is big enough to take the IO of a 512 bit bus, it would be foolish not to put it on. Thats not to say it will all be used, it could be some sort of redundancy. 8 64bit chunks tied to 64 rops, fuse 2 64 bit chunks and end up 6 and 48 rops as has been rumoured (I'm assuming rops are tied to the memory bus here). As to the number of stream processors, maybe they've gone for smaller less complex SPs so 4096 might be possible.
One can dream