What makes you say that the GPU is weaker than the 6670's GPU?
Radeon HD 7660D performs worse than the Radeon HD 6570.
Below is another review from Rage3D.com which basically compares the A10-5800k (Radeon HD 7660D) vs. the A10-5800k DG (Radeon HD 7660D + Radeon HD 6670 dual graphics), Core i3 + Radeon HD 6670 and lastly Core i7 + Radeon HD 6670. There are other configurations though.
http://www.rage3d.com/reviews/fusi [...] x.php?p=11
As can be seen, the A10-5800k performs worse than the A10-5800 DG, Core i3 + Radeon HD 6670 and Core i7 + Radeon HD 6670. The exception is Civilization V were all four provides similar performance. However, the A10-5800k DG does perform well.
If you continue to insist that the integrated Radeon HD 7660D is as as good as a Radeon HD 6670 w/ DDR3 RAM (probably about 10% slower than w/ DDR5 RAM), then you should back it up with some benchmarks.