Have you compared what's being reported in cgminer to the actual hash rate on your pool?
What you're seeing in your mining software is not your true hash rate.
Agreed - my miners are still showing the correct hash rate at my pool not the 'overclocked' rate and I left them running for a good 8 hours with these settings.
Depending on your host / set-up and the mining pool I'm fairly sure that there is a slight increase in the overall hash rate.
It is certainly not as high as the hash rate that appears to be shown in my original post, hence why I refered to it as 'an illusion'.
The actual speed is probably similar or less than that posted by Raptor2213
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Mine are using the OP's configuration (minus the custom cgminer) and are holding steady at about 397MH/s.
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The real question is can these settings generate more Accepted shares. I'm fairly certain that with some set-up's using stock block eruptors - they do, but again it is highly dependent on the host and the individual set-up.
Some of my own rigs are seeing more Accepted shares at various pools with a higher WU rate. I'll continue testing and moding the block eruptors and software. I'm certain its possible to get faster stock speeds with BE's, because of something else that I'm working on.