Heres the scoop, Ive done both a 3w white with a filter and a white K2 with the same filter. The K2 is the way to go to get the brightest purple. I run the setup off of the stock MR board. However, for the K2 to get to full brightness I used a relay to fully drive the LED. Doing it this way produces a much more satisfying purple blade.
Heres some pictures.
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