No it would not. The only reason it would if you got a really high end card (like HD 7950), then the CPU parts of the APU would be too slow for it. It's called bottleneck.
The AMD APUs were also built to support an external AMD card in Crossfire very well. However, there may be a limit to how fast a card will be supported; IIRC, this is pretty well documented in either the motherboard or processor docs.