According to the GWS site, the 8x4 on a 7.4v battery draws 10.4 amps, 13.4oz of thrust and 76.96 watts. That's assuming he's using the GWBLM001 motor which is what it looks like in his photo.
http://www.gws.com.tw/english/produc...r/gwblm001.htm
Just jumping to 8.4v would give 105.84 watts. That worth the extra battery weight.