richms -
I've been doing ome research into the CRI levels of different bulbs and their suitability. An interesting read is this article:
http://www.aquabotanic.com/lightcompare.htm
From glancing through it I think the point you made about the brightness relates to the efficiency of high CRI (90+) tubes. I believe that because they cover quite a high spectrum of light, the appearence to our eyes is dimmer than a low CRI bulb, but I think the most important aspect is the spectrum peaks - this article is quite good:
http://www.aquaticplantcentral.com/foru ... arium.html
The main point would be to see which peaks are being utilised by the buld, the most important being:
Chlorophyll-a: 430nm/662nm
Chlorophyll-b: 453nm/642nm
Carotenoids: 449nm/475nm
So, I assume that if you try and match the bulbs to cover this spectrum, you would have all your plants light needs covered.
p.s. Reading back what I've written, it sounds like I'm some cocky expert on the matter - I'm certainly not, please chip in with contradictions. I've just read a lot this due to an exceptionally boring week at work.
I'd love to hear peoples opinions on this.