This reminds me of people not believing the 4850 was as fast as the 8800gtx. Im going apples to apples, showing that originally the 4870 was 6% faster than the 260. Thats why we even have a 216. Then the 1gig 4870 came along, and took care of that. No clock speed increases, no other optimizations, just added ram.
Back to apple to apple, the 216 is only 24 shy of the 240 on the 280, thats 15%. Now, wheres this 20%? Once you go from a 216 to the 4870 1 gig? You automaticaally lose 6-7%?
OK, so, I think within reason, weve shown that the 4870 1 gig is = to the 216, which has 15% less available HW than the 280. As said earlier, it doesnt scale 100%, so, conclusion is 13% more in the 280, or, also, the 4870 1gig, which, as Ive already said, hasnt received 1 change to it, other than more ram.
Now, add faster ram, higher clocks, as per my earlier link of 950+, and youll see its quite possible for the 4890 to best the 285, and as jennyh has pointed out , itd make NO sense in it being just a lil faster, and not compete with the next higher card of its competition. The ONLY argument to that would be, if ATI projects the next series (5xxx) wont be fast enough to reconsider a newer purchase at the high end, which is possible, but probable?