cant set both shaders and threads. if you set the tc it ignores the shaders. if you set only the shaders without the tc if will auto calculate what the tc should be.
--thread-concurrency:
This tunes the optimal size of work that scrypt can do. It is internally tuned
by cgminer to be the highest reasonable multiple of shaders that it can
allocate on your GPU. Ideally it should be a multiple of your shader count.
vliw5 architecture (R5XXX) would be best at 5x shaders, while VLIW4 (R6xxx and
R7xxx) are best at 4x. Setting thread concurrency overrides anything you put
into --shaders and is ultimately a BETTER way to tune performance.
SUMMARY: Spend lots of time finding the highest value that your device likes
and increases hashrate.
It's an ambiguous misphrasing. When you set --shaders cgminer will automatically derive a thread concurrency value from that number but you can specify your shader count and following that override the auto-derived TC by setting your own value. If I set 256 shaders I get the same performance as setting 256 shaders and 1024 TC but a different performance if I set 256 shader and 512 TC, and if I only set TC 1024 I get a different performance as well, and different still if I set 128 shaders and/or 1024/512 TC.