Ugh, I don't know why people even use arbitrary numbers to describe the jump in visuals. Where do you even come up with a number like 8x? In terms of raw processing power I get it, but trying to turn the real world jump in visuals into a single number is meaningless. Because an "8x" increase in visuals has a different meaning depending on who you ask.
To actually describe the move from one generation to another, you have to describe the new technologies introduced, or the refinement of technologies that were already in use. For example, this gen we saw a large increase in geometry detail, the added use of normal mapping, dynamic lighting, significantly more real time shadows, more particles, more physics, better AI, etc. It's practically impossible to put those real-world improvements into some arbitrary number. A number doesn't describe anything.