That graph is very interesting, it clearly shows the
GT640 which happens to be a DDR-3 part being almost as fast as HD7730
GDDR-5.
I'll tell you a "secret", it has
33% more Texture units and double the color ROPs of HD7730.
When the game needs more pixel shader processing the higher Texture unit count will be faster even if you have lower memory bandwidth. That means that
HD7730 is Pixel shader limited in that games first and then memory bandwidth starved.
Also, it is clear that by only changing to a better architecture alone(GCN vs VLIW5/4) can give you another 22% more performance with the same memory bandwidth(HD7730 DDR-3 vs HD6670 DDR-3). Now add that Kaveri will have more Radeon Cores, more Texture units, newer Memory controller and HuMA than Trinity and we are looking for a nice performance gain. Also, Compute performance for Tessellation and other DX-11 features will be way higher than Trinity.
It would gain more with GDDR-5 for sure and it will be Memory Bandwidth limited but memory bandwidth is not everything, especially when your Graphics Pipeline is stalling.
Edit: HD7730 review link bellow.
http://www.tomshardware.com/reviews/radeon-hd-7730-cape-verde-review,3575.html