The figure of 7lb is for the up graded Formula more suited to the larger engines.
As with all things the plane was designed for an engine range and then people like me started putting OS91FX's in them so Modeltech up graded the plane. The wing is and control surfaces are very efficient allowing you to rolling harriers at near enough zero forward speed in no more than a gentle breeze.
Form more information and videos try http://www.ytinternational.co.uk/ and www.wmmfc.com