For an efficiency comparison between Sram S7 and Sturmey AW, you can reference this article:
http://www.ihpva.org/pubs/HP52.pdf
The efficiency of gears 5-8 on the Nexus 8 should correspond roughly to the chart for the Nexus 4. Gears 1-4 on the Nexus 4 should be derivable from the efficiency of gears 5-8 multiplied by an efficiency for the reduction gear, which I estimated as 0.95-0.98.