There are ways of reducing the amount of dope you need to finish the silk and hence the weight - it is not the silk which is heavy but the amount of coats of dope needed to airproof it. One is to use an airproof under-covering such as doculam as described in JMP and my posts above. The other is to employ a method of doping known as "meniscus doping". See http://www.vintagemodelairplane.com/...rs/Silk01.html
for a full explanation of this.
Obviously film coverings are very user friendly, they have their place and some are excellent (although there are some that are absolute
to apply) - but one thing that really pushes my "yuk" button is seeing scale models of aircraft that were originally fabric covered, especially World War One types, covered in shiny film