Are the latest generation dGPUs, such as AMD Radeon R9 290/290X, NVIDIA GeForce GTX Titan and GTX 780 fully supported by the GPGPU benchmarks?

Yes, but on such dGPUs, where clock boosting and/or throttling is used, it is very important to decide whether you want to measure the absolute maximum attainable performance or average performance. If you're interested in maximum performance, only start AIDA64 GPGPU Benchmarks when the video card is cool and power limits are set to a relaxed value (AMD PowerControl). If you're interested in average performance, then make sure to disable the CPU benchmarks, and execute the GPU benchmarks at least 10 times consecutively, so that the video card heats up.