Believe i saw it on anandtech or tomshardware in an article, but heres from wikipedia "Up to 17% more CPU performance clock-for-clock compared to Lynnfield processors:" and read somewhere that Lynnfield was 5% faster than Nehalem clock for clock. So i added the two together and i got 22%. Im going to try and dig around see if i can find that specific article i saw. But still 920 is plenty fast and is a great match for the brand new bulldozer cores.
If you google "sandy bridge vs nehalem clock for clock" you will find my 22% is about right.