I apologize to butt in to the thread here, but with the higher K bulbs; have you guys looked into the aquarium trade?
They began using the double ended MH bulbs back in the mid to late 90's. However, whereas in this industry they are called "DE"; in the aquarium industry they are called "HQI" (halogen quartz incandescent). 6500k is about as low of Kelvin as people tend to run on aquariums and you can get the HQI's in 10K, 12K, 15K and 20K.
I believe they top out at 400w for the HQI metal halides for that trade though. (also can get 150 and 250's).
In the aquarium trade they were "advertized" to provide better penetration / intensity for less watts, but they had UV and needed to have a glass lens (with aquariums). This is because UV is removed with the "outer" glass that mogul bulbs have and HQI or DE's do not. UV comes from the Mercury I guess if anyone cares there xD.
sorry again, just wanted to say to check around in the aquarium trade (if ye' haven't.)