Snapper,
I think going any lower then 5KK whould have the same effect as going with the higher kelvin bulbs. There just isn't enough balanced red and blue. You can get Venture 5.5kk pretty cheap. Although the hydroponics guys like the lower kelvin and go for warmer bulbs around 3KK. I have a new Sylvania 175 mogul you can try and see if it has any impact on growth.
Maybe do an experiment for the club. :D