From what I've seen so far, Nvidia-cards from Maxwell, Kepler and even Fermi-generation can profit marginally from carefully administered Asyn Compute. This margin and the amount of care that has to be taken are way more pronounced and relaxed respectively on AMDs GCN hardware. My working theory is, that the amount of work thrown at the GPU from multiple threads lets some buffers overflow, sometimes leading to adverse/unwanted effects like decreased performance. This is true for GCN cards as well, but the amount of work in order to do that seems stupidly high.
In other words, a workload carefully optimized for one arch might overload the other, which in turn would not really need to have AC turned on in the first place.
Anyone remember "Ice Storm Fighers"? A futuremark-built multithread-demo for Intel processors a decade or so back. You could dial the amount of parallel simulated enemy vehicles and gain performance from multiple cores in almost a linear fashion - until you hit a point where perf would tank. Same principle here. Might have a nice picture lateron.