The chips don't even exist yet. Assuming a 256-bit bus, you'd need 4Gb chips to put 8GB ram on it. GDDR5 4Gb chips are not currently available, and are not on any roadmap I know of.
I believe Hynix is supposed to start production of 4Gb modules this quarter. I can't find the link, but I've read it on this forum.
My guess going to 4GB required that. With 4Gb modules, it's only 8 chips.