Difficult to answer that one as the propogation of radio waves without a site survey is impossible to predict. each 3dbi will equate to a doubling of power, so you have a very definate increase in power there, but it depends on the footprint of the aerial as to where all that extra power will go. an omni aerial isnt trully omni directional, and will perform better in certain directions than others, with the downwards direction possibly being far less powerful than outwards. hence raising the height may not be beneficial. you need to look at the footprint drawing for the individual aerial you are using to determine this.
A directional aerial may be a better option as its footprint will be more focussed, and therefore stronger signal in the area you need it.
Also, remember that sources of rf will interfere with your wifi signals too, and increasing power will not resolve isssues caused by rfi (eg.power leads, televisions, flourescent lighting, to name but a few)
Edited by Fozzies, 04 November 2011 - 12:49 PM.