I have a similar configuration: AMD A8+ HD 8750m.
These games you listed are disabled by default with Dual Graphics, don't know why but even though they have a crossfire profil and are DX11, Dual Graphics won't work.
I tried to apply a different crossfire profile for BF4 (BF3) and then, Dual Graphics worked and it gains 10 FPS more than with the HD 8750m alone, BUT it's a horrible stutery mess. It seems AMD hasn't optimized Dual Graphics with VLIW4 and GCN yet and that's poor because they are wasting so much potential. For example my 3DMark score with Dual graphics is on par with a GTX 760M but I can't use it because it's a stuttery mess. If you want to check if the game runs in Dual Graphics mode, you can enable the Dual Graphics icon under the AMD Catalyst System Tray Menü->Dual Graphics-> Options->Show Icon
But they announced with Beta 14.1 Frame Pacing with hopefully new Crossfire profiles optimized for Dual Graphics, maybe that will finally do the trick. Let's hope!