there are so many things that come into play when doing this,sure ive had great returns off plants ive tipped and trained,BUT,it all comes back to time,i havent done this outside using the sun but under lights you can get the big yields it just takes alot longer and imo isnt worth it unless your grow space is limited and your time isnt.
as for the yield being proportional to root mass,not always the case,the plants i grow in my NFT system usually produce an average of 2oz each and the root mass can be nothing compared to 2oz plants ive grown in soil before.there are just so many variations when growing weed.