AMD has kept the wraps on this one tight, however a document shows that the Tonga based Radeon R9 285 will be released onb the 2nd September, 2014. Tomorrow August 23rd AMD is celebrating 30 years of graphics and gaming. On that day, AMD will share "partial" details of the card. 

The GPU has 2048 GCN stream processors running over 32 compute units. The chip would have 128 texture memory units

The block diagram originates from a FirePro W7100 professional graphics card presentation which is based on "Tonga." The W7100 uses 28 of the 32 available CUs and as such you can easily recalculate the true number of shader processors. Tonga will be tied to a 256-bit wide GDDR5 memory interface

The initial consumer graphics card based on Tonga will be the Radeon R9 285 which is expect to have 1792 activated stream processors making it perform at GeForce GTX 760  and Radeon HD 7950 level performance but with  better thermals and lower power consumption.

The R9 285 is designed to offer performance roughly of the Radeon R9 280 but is more energy-efficiency.