I believe that a higher boom will allow you to plane earlier because in being higher allows you to pull down more on it, transferring more of your weight onto the mast base and off of your feet. Getting weight onto the mast base helps keep the board flat and makes it more efficient through the water, ultimately planning faster. Also, in getting weight off of your feet, especially on smaller boards, you're less likely to make the tale sink slightly, which would add resistance and make you start planing slower.
This discussion about fin size/weight also happened earlier on the forum and I believe that the conclusion reached was basically that fin sizes were more closely linked to sail sizes as for the same wind a heavier sailer will need a bigger fin not because he is heavier but because he is using a bigger sail.
This is a great article about how putting more weight on the mast base is beneficial to your overall sailing:
Hope this helps.