The 512sps GCN card on the desktop is Radeon HD 7750 that is GCN1.0, consumes around 55-60W on gaming and performs around a GTX750m-760m.
GCN1.1 revision improves sightly the efficiency of GCN archs and GCN1.2 improves the performance/SP of the cards in a bunch of gaming situations.
7790 consumes ~20w more than 7750 and uses the same die area as 7750(that is a partially disabled 7770).
So, do AMD could put 768GCN1.2 sps on the next APU?
Do be careful. Lots of people are comparing performance per sp from tonga to taheti, which had the lowest perf/sp of all gcn 1.0 products.
http://www.hardware.fr/articles/926-24/tonga-vs-tahiti.html
Hell, that's <10% gain per sp from taheti to tonga (GCN 1.0 to 1.2). Not saying that this test isn't flawed but that tonga, at least in its neutered form needs a little more to make it competitive.
http://www.techpowerup.com/reviews/Sapphire/HD_7790_Dual-X/26.html
1200p
7790 - 95%
7870 - 141%
Same clocks, 1280 vs 896 sp. 43% more shaders on the 7870, 48% more performance (Pitcarin has more bandwidth: performance delta grows from 720p to 1440p). Bonaire does so with significantly greater efficiency though.
Bonaire is 160 mm^2. Cape Verde is 123 mm^2. Pitcarin is 212 mm^2.
7790 is significantly larger than a 7750.
More than 512 shaders only makes sense if HBM is a go. As well, 768 sp is too much for 35W mobile so AMD would have to either disable a bunch of shaders or produce two dies (512 barely fits in a 35W window).