For all tests we used AMD A10-5800K with HD 7660D graphics engine. From the whole 8GB of system memory IGP got only 512MB. This memory was allocated by system and we did not have influenced this.
IGP memory speed mirrors system memory speed. If you wish to increase memory bandwidth, and in turn IGP performance, we can do that by overclocking system memory. AMD advises users to use system memory running at 800MHz, or up to 993MHz, which is the official maximum supported AMD's APUs. GPUZ screenshot confirms that CML8GX3M2A1600C9B works at 800MHz (1600MHz DDR3).
As you can see, the results of memory overclocking show on the bandwidth numbers.