It always sounded stupid to me that we should not use all threads available. After all it's hard to expect that a cpu running at 100% load can't deliver more power than 50% loaded.
Therefore, to convince myself, i've conducted a test of 300 games with Stockfish 5 against itself using cutechess.
Although 300 games are usually not enough to speculate precisely a superiority, they show 4 ht cores on 2 physical cores can't beat 2 cores. The difference was only 12 ELO in my case.
Besides 4 ht cores are able to hit 2.8Mnps while 2 cores can only hit 2M.
There are 40% more nodes analyzed but they did't lead to better moves...