1% is what W1z got on his with stock voltage. He was able to push it to 1000MHz after upping the voltage, but even at 1000MHz I don't think the HD4890 can compete with an overclocked GTX275...
Most reviews show that a 1000mhz 4890 can equal a stock GTX285 in a couple of things, it loses out a little in many but betters it in a couple, there are plenty of reviews out there of the Sapphire Atomic, so I suppose you could say, for someone that dont overclock, perhaps the high end 4890's with good factory overclocks may just be the better buy, if you overclock perhaps not, again as you mentioned, it's down to cost, would I pay a premium over a GTX275? probably not. It's the luck of the draw on how these things will overclock out of the box, thing is, before I got this card to compliment my GTX275 which is in my Yorkfield rig, I read 5 reviews on the Toxic, every one got the core to at least 1020mhz, I spose I got lucky at 1050mhz, W1z's sample obviously wasnt so lucky!
The thing is, W1z on his first review of the "standard" 4890 clearly said, when you cut through the initial ATi hype, most 4890's wont actually hit 1000mhz, at least not without lots of ugly volts being put through them and it was clear to me all along that in stock guise a 4890 was a bit slower than a GTX275 which is why I got the 275, but if you can get them to 1000mhz, baring in mind there are no voltage increase options for the 275, then you do have one fast single GPU on your hands, here is one of those reviews of the Atomic at stock 1000mhz and overclocked, mixed results.
http://www.overclockersclub.com/reviews/sapphire_hd4890_atomic/13.htm
