Isn't it a bit late to target the GPU towards that? Wouldn't specs already be nailed down by now?
Yes and no. The target for performance may have been set and current dev kits are near representative of that kind of performance being simulated via off the shelf parts currently available.
The final specs are a part of a road map of hardware development which currently either exist or are planned for production next year. Thus, say, if they have targeted the GPU tf performance to be 2.2 then they'll simulate it with whatever card is available today for developers and wait till say the 8850 or 8770, whichever yields the same target figure with better power efficiency.
AMD's mid range cards were out late Q1 this year and so it makes little sense for a console coming out in late Q4 2013 or Q1 2014 to use tech that'll be nearly 2 years old.