Fusion APUs eliminate the PCI Express bus which bottlenecks many GPU applications. However, integrated GPUs in APUs are less powerful than discrete GPUs, and while the upcoming APUs will rely on a unified memory, the first APUs still have a distinct GPU memory partition. Hence, it is worthwhile to investigate the applications, as well as problem sizes, for which the GPU part of an APU may outperform a discrete GPU. In this talk we assess the relevance of Llano and Trinity APUs for scientific HPC via hardware and applicative OpenCL micro-benchmarks. We present detailed measurements of CPU-GPU data transfers and performance tests of highly optimized 3D finite difference stencils (a synthetic one and a more realistic and complex one for wave propagation simulations) depending on the frequency of snapshot retrieval. Our results show that integrated GPUs of upcoming APUs can outperform discrete high-end GPUs for medium sized problems with high communication requirements.