I have an interesting phenomenom with neoscrypt, on 280x.
With intensity 13, I get about 298kh/s stable. If I then lower intensity to 12, hashrate rises to 305kh/s and stays there.
However, if I start sgminer directly with intensity=12, hashrate is only 270kh/s or so, and does not rise any higher..

So it need to go through intensity 13, then lower to 12 to get max speed.
This is with latest sgminer develop version, Wolf's kernel, catalyst 14.6.
In my case at i13 I have lots oh HW errors, but after lowering intensity to 12, hashrate remains the same as in i13 but no more HW errors...
But after a few minutes hashrate starts to slow down to i12 expected speeds.