So i have the same problem with low GPU usage and FPS drops, with my I7 6700 3.7Ghz, R9 290 4GB , 8GB RAM and SSD. At some point the FlipQueueSize=2/3 has improved the fps a bit as well as the RenderAheadlimit=3, but still there is plenty of moments with 60% GPU usage and 40 fps. That`s really irritating.