Our tests showed clearly that the AMD Dual Graphics technology really boosts the performance in games. On most of the tested games (as well as on the 3DMark benchmarks) the performance with the technology enabled was superior to the obtained with the integrated video alone or with the discreet video card.
In two games we ran, however, there was no performance difference. Our guess is that, in those tests, the performance was being limited by the CPU, which is not a high-end processor, after all.
Another important detail that was clear is that the integrated GPU of the A8-7670K is more powerful than the Radeon R7 240 video card we used. It makes sense, since the technical specs of this graphic engine are slightly superior to the ones of the used video card.
So, if you have an A8 or A10 CPU, there is no sense in buying an entry-level video card and disable the integrated video of the processor: you will be burning money and maybe even reducing the video performance of your computer. However, you will get a performance gain in most games if you buy a video card that can be used together with the integrated video of your processor using the Dual Graphics technology.