The 8% for sugarcane is when they're at the peak of their abilities; you'll need to average over the entire lifecycle to work out what humans can do with a plot of land, and that takes you back down to a couple of percent again.

Re: Hall and Rao's Photosynthesis, p.67 of the fifth edition shows efficiencies of up to 12% in certain wavelengths in Chlorella, but the operative word is "in certain wavelengths". Hall and Rao are all about the photosystems and the chloroplasts, but that's not a measure of the final productivity of even an ideal farm.