For most use-cases this setup will act like a pure dual-channel setup, considering that most users rarely use all the available memory. It's even better if the memory dedicated to the integrated GPU comes from the lower addresses where it can get the full bandwidth of a dual-channel setup. 3/4
I wonder what AMD does. I noticed while testing memory on a laptop with memtest86 that asymmetric setups have less bandwidth than symmetric ones, which suggests that addresses are striped between the channels, contrary to what Intel does. However I couldn't find any data confirming this in their hardware manuals. But I didn't look too hard and it might not even be mentioned. After all memory channel arrangement is generally neither user-controllable nor user-visible. 4/4