Well I think it is (in theory - with ideal conditions) possible to get 1 kilometer Wifi range with ~8,5 MBit/s but there shouldn't stand something in signal's way. You have to build a 'Cantenna', connect it with your router and fix it on your roof/ window. I found a very good and technical article here about building such an antenna by Leibniz University of Hannover (Germany). It also contains a calculater for the perfect can-diameter and important information. You can find other important informations about the right cable and signals-characteristics here (german text)