Further to my informal testing of 3S and 4S batteries with the same prop I did a test with 9x5 3-blade props (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=...) and 11x4.7 2-blade props (APC SF 11x4.7).
Test procedure: using freshly charged batteries I took off and hovered at ~2m for 60 secs and then landed.
KDA20-22L motors from Hobbyking
2 x Super Simple 25-30A ESC's from Hobbyking
2 x Hexfet 25A ESC's from Hobbyking (2 of 4 were DOA)
3S 3000mAh Turnigy 25C
Va - 11.75V, Pa - 161.6W, Aa - 13.8A
Va - 11.89V, Pa - 150.2W, Aa - 12.7A
11x4.7 uses ~11W less power and ~1.1A less current on average for the 60 second flight than the 9x5
Full graphs below: 9x5 first.