This method actually caused a decrease of 30%. I went from 26.81 MHash/s down to 18.71 MHash/s.
I have used this with a Radeon 5830 card. from 305MH/s to 315Mh/s
Here's some proof.
new kernel
http://imgur.com/JcMmZold kernel
http://imgur.com/YrRuLI was using it with a Geforce GTS 360M. Perhaps it only works with ATI cards?