Thanks for the overdriving idea -- I am experimenting with the use of this idea today. Earlier this afternoon I moseyed on down to our local "Home Depot" store and purchased some Shop light fixtures for $7.97 each (this is much cheaper even than purchasing replacement ballasts alone). They are pre-assembled and I spent most of the day disassembling every other one and them reassembling each pair into one overdriven dual-lamp unit, rewiring each ballast to overdrive a single lamp. I removed the ballast from one, rewired it to power a single lamp vs. dual lamps and inserted it into a light fixture where I had already rewired the on-board ballast and one lamp.
It is easy to see that the lamps are burning much brighter. I am curious to see how long the lamps last when overdriven like this. I am much more curious to find out how the plants respond to the increase in light.
The fixtures come without lamps. Fortunately I had earlier located a clearance sale on Philips F40T12/Contractors CW/Alto lamps, 30 lamps for $20.00 or about $.67 each. My stockpile of these is quickly diminishing so I am anxiously looking for other sales of similar lamps.