Last results , tests , conclusions , on my n9 :

1350mhz , 1.325v , sr off - Rock solid , I think this is the lowest voltage for 1350mhz .

1350mhz , 1.325v , sr on - Unstable , rebooted after 3 minutes
1350mhz , 1.3125 , sr off - Pretty stable but I got a reboot after ~12 hours .

So far I found perfect voltage for 1350mhz , but I don't think 50mhz makes a visible difference so I will try with 1300mhz and lower voltages .

I feel sorry for the guy who fried his chip , I also stopped trying high voltages and frequencies that my device can not handle . I've tried max 1450000v via xterminal but is still very laggy and unusuable after 1370mhz . N9 is such a "fragile" device , N900 is like a rock , there are very few permanently bricked N900s reported .