AMD R9 290X?

From the scrypt readme

Quote

--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.

Quote

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TUNING AN AMD RADEON 7970
Example tuning a 7970 for Scrypt mining:

On linux run this command:
export GPU_MAX_ALLOC_PERCENT=100
or on windows this:
setx GPU_MAX_ALLOC_PERCENT 100
in the same console/bash/dos prompt/bat file/whatever you want to call it,
before running cgminer.

First, find the highest thread concurrency that you can start it at. They should
all start at 8192 but some will go up to 3 times that. Don't go too high on the
intensity while testing and don't change gpu threads. If you cannot go above
8192, don't fret as you can still get a high hashrate.
Delete any .bin files so you're starting from scratch and see what bins get
generated.

First try without any thread concurrency or even shaders, as cgminer will try to
find an optimal value
cgminer -I 13

If that starts mining, see what bin was generated, it is likely the largest
meaningful TC you can set.
Starting it on mine I get:
scrypt130302Tahitiglg2tc22392w64l8.bin

See tc22392 that's telling you what thread concurrency it was. It should start
without TC parameters, but you never know. So if it doesn't, start with
--thread-concurrency 8192 and add 2048 to it at a time till you find the highest
value it will start successfully at.

Test iteration 6 on 128 MiB of memory on device 0 (Hawaii): 0 errors so far
Moving Inversions (ones and zeros): 0 errors (0 ms)
Moving Inversions (random): 0 errors (16 ms)
Memtest86 Walking 8-bit: 0 errors (31 ms)
True Walking zeros (8-bit): 0 errors (16 ms)
True Walking ones (8-bit): 0 errors (15 ms)
Memtest86 Walking zeros (32-bit): 0 errors (78 ms)
Memtest86 Walking ones (32-bit): 0 errors (63 ms)
Random blocks: 985 errors (0 ms)
Memtest86 Modulo-20: 0 errors (171 ms)
Logic (one iteration): 0 errors (0 ms)
Logic (4 iterations): 0 errors (0 ms)
Logic (local memory, one iteration): 0 errors (16 ms)
Logic (local memory, 4 iterations): 0 errors (0 ms)