So...with Julia4D stress test I've got around 17%Bus usage on 1 card and 27%on the other...is that normal ?
And when I try another benchmark,well...my PC just shut down !!! No blue/black screen, just off... this is wowering me right now...
Some bus usage certainly is normal, this may vary widely, depending on the application.
What I was warning of, is EXCESS bus usage due to inter-videobuffer communication (figuratively speaking), when monitors are connected to different cards.
So you want to compare bus load under these different circumstances (both on card #1, both on card #2, each one on one card, etc.).
In your case of 17% on one card and 27% on the other, what comes to mind is that your second card seems to be seated in the x8 PCIe slot, while the first one is in a x16 slot. You might want to experiment with different slots for your second card.
According to this document, #2 and #5 are the x16 slots:
http://h20331.www2.hp.com/Hpsub/downloads/Optimizing_PCI-Express_on_Z800_3-18-09.pdf
The patcher comes back with a successful, the driver installs in safe mode (but no certificate), I can reboot and everything appears to be working properly but there's no SLI option.
From this summary I take it that your system still loads an unmodified driver.
Device Manager → Double click your main display adapter → Tab: Driver → Driver Details
Take note where the active nvlddmkm.sys is located.
Go to that location and open the file properties of nvlddmkm.sys→ Tab: Digital Signatures
An unmodified driver would have 3 certificates listed, one from MS the rest from nVidia.
Your modified driver however should list the certificate from DifferentSLIauto.