I have 2 x 3 ft flouros. in the factory fixtures over my 125. I modified them with elec. ballasts designed to drive 4 x 4ft lamps, and am using all the outputs of each ballast on each 3 ft. bulb.
That would equal 4x overdrive on a 4 ft bulb. Maybe a little more on the the 3 fts.
I don't have plants yet, as this is a test of light output vs. lamp life.
They ARE noticeably brighter, as well as warmer to the touch, but so far (about 5 mos. now) they have had no issues other than growing algae in the tank more than at normal output.
By standard fixtures I mean the normal plastic single bulb strip lights that come with tank packages.
As far as your 1x overdrive question, I think its a moot point.
1x is not overdrive, it's normal output. Can you use a 30 watt bulb with a 40 watt ballast ? sure, but it won't be overdriven I don't think.
Most electronis ballasts "sense" the bulb attached to the output and will "compensate" somewhat for a lower wattage bulb by delivering less voltage to the outputs.
Where overdriving comes into play is when you use a multi bulb ballast to drive a single bulb with multiple ballast outputs.
That'll Do Donkey, That'll Do. - Shrek
Filstar Pimp #49