I had just upgraded to an i7-3770K (8 threads, 4 cores) from a Core 2 Quad (4 threads, 4 cores). I did a POV-Ray render several times using 1, 2, 4, and 8 threads. 2 was nearly double the speed of 1, 4 was nearly double the speed of 2, but 8 was only about 15% faster than 4.
To ensure I wasn't bottlenecking RAM at that level, I tried again with 2 threads, but forced both threads onto a single core, and it was only 15% faster than 1 thread.
That was all the proof I needed of how you really can't treat two CPU threads as two cores.
That said, I've never personally found an instance where allowing a processes to span all CPU threads actually reduced performance, and I'm not sure I've ever seen a real-world case where it does. It's usually something contrived.