Groestl-myr low performance on 280X

[MaxDZ8]

Reported by user A, running M8M and user B running M8M kernel ported to a special sgminer build.

User A reported lower performance, in the order of 25% lower.

User B reported performance cut in half. It is known he is using an old driver.

This must be better investigated by users themselves as I have no way to reproduce.

[qwep]

where you can take a miner sample

[MaxDZ8]

CodeXL has a remote profiling operation but I'm not very eager to do this kind of testing in arbitrary environments. The drivers are brittle even in local operation, I cannot imagine what could happen in remote.