How Much, I have added a No5 section to the base of the 5 section Ungers (to make it a 6 section) by making new holes at the opposite end of the base after removing the handle with a heat gun. Make a paper template and copy the sqare hloes then drill out round the lines with a small drill, knock out the waste and file to the same square shape. Fit in the green insert washers ( you can get away with just two which gives enough grip) and fit an extra No5 cone, then simply slide in a number 4 section upside down.
If you want to go to 7 sections you need a 15" piece of the No 5 section which you slide into the base of the converted No5 then add another new No5 section.
If you want to buy these sections I have a few spares.
In practice I have been as high as a No8 section but I advise that the max is 7sections or 42ft and even then its hard work.
As you have the time, keep an eye on my Featherlight pole topic over on Matts site, I about to stick my neck out and reccomend a pole.
http://d.co.uk/phpbb/viewtopic.php?t=194