SPCR is a review site pretty serious about their methology; their newest heatsink reviews don't have °C/W figures due to difficulty measuring the power output of socket 1366 CPUs, but here's a mid-2009 review of a huge heatpipe/tower heatsink: http://www.silentpcreview.com/scythe-mugen2

Page 6 states °C/W values of 0.29 down to 0.14 depending on the amount of air pushed with the fans. SPCR's test fans are pretty low-flow to begin with (47 CFM/1080 RPM at 12 V), so you could probably go slightly lower by using real screamers.