Bright as a stock MR may be on Lithiums, a Lux III conversion of the same color will always be brighter with the same batteries. And if you use Ultras blades, it will be just about as evenly lit. I have a Luke ROTJ stock MR and a Lux III Ultra stunt, and with fresh Lithiums in both the Lux III Stunt is always brighter.
And if you want to get a purple in your conversion, you'll have to get a white LED and use a purple filter. I wouldn't recommend, as brightness will suffer.
Hope this helped,