Overdriving shorten tubes life strongly. Up to 1/10. And it reduces energy efficiency too, although little (about 15%).

I think is better (and especially, cheaper in the long run) to put twice the tubes than to run them twice the power. If you do it because space limitations, go to Reflex tubes. You can mount them almost touching themselves because they have an internal reflector with a 160ΒΊ window. If there is no space for that, yep, overdrive is the solution, but its an expensive practice on the long run.

I wouldnt advice to do it except as the last option.

RackitMan, you are right on the money. Coolwhites (~4000-4500K) often are the tone which emits more between 500 and 600nm, the range in what we are less interested. Ive always used fluorescents to grow and i avoid 840. I use 865, 830 and 827.

But daylight tubes (typically 6000-6500K) emits about 40% of light on blue (simplifying the 400-500nm range) while warm whites less than 20%.