Ten percent seems perfectly logical.
No one has been able to scale well once they get into the upper range.
The radeons 4xxx are already highly clocked. I would not expect a 20 percent increase in performance.
I can imagine if driver performance can be brought up, I can see 20% increase in April. But since this driver improvement is extendable to the 4870. It makes the 4890 a totally unneeded product. Especially since it sounds like it will be sold for 20 percent more money. I doubt these will come with 1gb of ddr5 ram at the predict price.
Off topic but..... putting signs like banana's where the letters are for a curse is still swearing(which is far more offensive than anything in OBR post) and far more snarky. If its directed at anyone, this is still a personal attack.