If the memory bottleneck can be removed (with HBM, for instance), theoretically an APU can be faster than a similar dedicated GPU because the communication between CPU and GPU is better?
Sorry for the non technical words.
Thanks for the answers guys, but I was trying to compare more in line with ShitaiDK´s thoughts: not absolute performance of APUs in the future (nodes, TDP, etc.), but it´s relative performance in comparison with CPU+dGPU in gaming or GPGPU scenarios without the memory bottleneck.
It isn't so much that the GPU and CPU can communicate faster with dedicated memory, but the GPU has more bandwidth which is needed for good gaming performance.
As an example, my Radeon 7970 with a modest bump on the memory speed (1430MHz) and the 384 bit connection to the GPU provides
276GB/s of bandwidth to just the GPU, the CPU gets none of that. That is 3GB of GDDR5 dedicated to just the GPU with that amount of bandwidth. (Keep in mind my 7970 has 2048 GCN cores running at 1030MHz -a slight overclock-, a high end AMD APU has 512 GCN cores running at, I believe up ~960MHz... more powerful GPU's need more bandwidth to perform up to what they are capable of.)
My CPU has a 128 bit connection to the memory (2 x 64bit channels of DDR3 memory) running at ~1760MHz providing something like ~
23GB/s of bandwidth, just to the CPU. That's the problem with APU's, both the GPU and CPU have to share that DDR3 connection which provides significantly less bandwidth compared to a discreet video card with it's own onboard memory. So making the APU's GPU portion much bigger and more powerful won't get you much in a lot of gaming situations because the memory bandwidth will often limit performance (and remember the CPU also is using some of that memory bandwidth).
With HBM or other high speed memory (the PS4 uses four DDR5 channels for a 256bit connection of fast memory that the GPU and CPU share in the APU, as an example) performance can increase and keep up with a more powerful GPU core, it can give it the bandwidth it needs.
So yea, as memory technology increases we'll see better and more powerful APU's. Right now I think they've gone about as far as they can go on dual channel DDR3, more bandwidth just isn't there right now.