It depends, with a slightly lower clock rate (918Mhz) than the 280 (933Mhz) and a 256Bit Bus Tonga (285) performance is from the same as the 280 and upto half way between the 280X and 290.
With a 1000Mhz clock and 384Bit bus it might sit around the 290.
It will be a total fail if they do that, the 290 is already a pretty efficient GPU at 230/240 Watts, the same as a 7970 GE.
Tonga will only take about 20 Watts off the 290, 10%, and with less V-Ram, 3GB.
Tonga is an utter fail, no one buys them as the 280/X is just plain better value with similar performance and more V-Ram.
The only reason Tonga exists on the retail market is because they are Apple Salvage parts that didn't make the grade.
I doubt it.
Maybe the 370X