BTW, I notice you tend to deck out your test systems with pathetically low amounts of RAM. When running extremely demanding tests like Unigine Heaven, isn't it safe to bet that they will be demanding at least that much RAM from the system? For all sorts of caching, textures evicted from GTT, etc.
And let's not forget all the overhead of running Ubuntu as a base...
What I'm saying is, make sure the system isn't swapping while you're doing these tests, or the numbers will be artificially deflated regardless of which GPU you're using.