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The Astronomik MC clear filter will not have any impact on RGB images and will still colour balance fine in an LRGB composition. The only time where you'll have issues colour balancing is when the RGB set are not UV/IR blocking. The luminance does not matter. You'll certainly get benefits image with a luminance filter on galaxies and widefield work. Its hard to beat a narrowband Ha filter for nebulae type targets. Chris Schur presented an interesting slide deck at AIC2007 regarding Ha imaging of galaxies - pretty cool.
I find it interesting that you can make a sensitivity comparison between the MC and UV/IR blocking filter. How have you determined this? Can you share unprocess fits files between the two filters so we can measure star intensity values?
Seriously, the difference between the MC and UV/IR filter from a sensitivity perspective would be minimal to none. We are still talking about visual wavelengths here. Sure the characteristics of the ST7 chip will allow you to go near infra-red, but you really need to use NIR filters to get any benefit. In fact, if you're using an optical design that uses lenses, you'll struggle to get clear NIR wavelengths due to the optical coatings. This is the reason why most (if not all) research is done with only mirror based instruments such as RC's as they transmit all wavelengths equally.
Personally, I'd also question the benefits of the Astronomik MC filter in comparison to unfiltered. Unfiltered would probably be a better option. There is also no reason filters need to be parfocal either. Sure it makes imaging a little easier, but if you're using FocusMax its easy to determine filter offsets for fast focusing.
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