Looks like unique to the card/system/magic. Check LTC hardware comparison wiki page.
On one of my rigs - 7950 44000 works best, 6950 - 12672.
But was helping a friend to setup his rigs, those values were not working at all..
I've read somewhere thread-concurrency should be n*number_of_shaders, no idea if it is true, but I would start there.