I forgot to post the reply to my email:
We recently became aware of a regression that prevents the GeForce GT 240, and possibly other cards (GT 240 is the only one we're aware of right now) from raising its clocks to the maximum performance level. The regression window you report (304.22 good; 304.37 bad) matches our observations for this issue, which might be responsible for the poor performance you are experiencing. This regression is currently under investigation.
Can you confirm, by monitoring the "PowerMizer" page in the nvidia-settings control panel, that your GPU does not raise its clocks to the maximum level with 304.37, but is able to do so in 304.22?
And after confirming it:
Yes, that does sound like the PowerMizer issue we're investigating. Thanks for confirming; for your reference, we are tracking this as NVIDIA bug 1038281, in case you want to make inquiries about this bug in the future.
So, no, it has nothing to do with Sync to vblank (wich i had enabled before this release because i want it)