Thanks for the replies, i tried both uncore x1,5 and x2 and also qpi/vtt from 1,2/1,25/1,3.
Mysteriously it started with 1600mhz now this morning, with the multi set to 23x.
But somehow the timings got really bad, im only able to get it stable at 9-9-9-24,
the lowest i can start it is 7-8-7-18, but gets me alot of explorer errors and bluescreens,
same with 8-8-8-20, increasing ram voltage didnt help either.
1,5x or 2x uncore and different qpi/vtt settings didnt affect the stability at lower latencies either,
so its running stable at 1600mhz (9-9-9-24) with 1,5x (2400mhz) uncore and 1,2 qpi/vtt for now.
Somehow i feel im better off with the 1333mhz (6-7-6-18) setting, if im not able to get better latencies at 1600mhz.
Also i read in a guide, that its better for westmere to set the uncore to 1,5x, than 2x like for bloomfield,
not sure if it makes any difference.