Observe what is happening. Are you caching the BVH the majority of the time? If so, render right after for a faster time. Also, make sure to change the tile size to something that will work better with the CPU. Since the i7 4790k's power is in the single-threaded performance, I would try 16x16 threads, so it tears through them like butter.
Also, check out task manager, and see if the CPU is running 100% the entire time. If it is not, then the threads could be causing some issues. Because of your current system issues, I would make sure that they FSB, CPU Multipliers, and HT speeds are exactly how they need to be for that CPU. There's a good chance that it's set lower than it should be.
As for the thread count in the file, as long as it's set to "Auto-Detect" you should be fine.
If all of that is good, I would find out exactly from those 4790k owners, and find out what they did. Also, try the test with the older version of Blender that was used for those tests (2.72 I think).
I hope that helps something.