"what is the purpose of overdriving?" -
Like you said - you can only fit so many plants under one fixture. After that, in order to get more light on them, you need a higher output fixture.
Although readily availible, these ballasts are much more expensive.
2Xoverdriving produces a higher power factor using cheap electronic ballasts. (reportedly 70% more light).
The link I posted above, (Tue feb 21), I think will take you to the thread you mentioned. There is also a 2nd thread w/same title + cont... if you want more info.
I did this with two of my fixtures, and while I don't have the equip. to verify a 1.7 times increase in output, I can say they are brighter.
I bought another fixture ($7.98) with the intention of doing the same thing, but have decided to use it as is, for other considerations (100% more light vs 70%, and twice the flexibility for other uses, as I can better use an additional fixture than to make the one I already have brighter).
As you also mentioned, with proper care and attention, standard fixtures produce good results.
I should also note that while using a product in a manner other than intended by the manufacturer may still be well within the design spec. of the components, the user assumes any risks by making modifications.
Link to VHO high output flurescent lighting:
http://www.marinedepot.com/aquarium_...dex.asp?CartId