I've done some research on the application of LEDs to aquariums, and this is what I've drawn from my reading related to light in general.
Lumens is a measurement of visible light intensity, meaning it takes into account wavelengths across the entire visible spectrum. This is useful when talking about task and home lighting applications... HOWEVER, PAR or Photosynthetically Active Radiation (related to PUR or Photosynthetically Useful Radiation) is the value that is important here, because this equates to how much of the total light output will actually excite chloroplasts in plants (including symbiotic algae).
So, what we need to know here is how much of the total 4200 lumens that a 30w xenon bulb (*edit* it was pointed out that a xenon bulb is the same as a MH but with xenon, rather than argon, as the starting gas [Thanks dabandit
]). This will show us just how useful these lights are in reefkeeping.
I'm not sure if K ratings equate to nanometers, as all discharge-type lighting systems including LED and arc lamps produce 'peaky' light that when analyzed by a spectrometer will show the true wavelengths of light produced. I think that Kelvin is an overall measurement of colour temperature as humans see it, rather than a function of the actual radiant energy produced by a given light source. That being said, a higher K will mean that the general wavelengths of light produced are further into the blue end of the visible light spectrum.
I think that this could be viable, since you would have a hard time finding any manufacturer of 30w MH lighting systems. However, when you start talking about a unit with 150w of xenon lighting (5 bulbs), what's the point? I should think that it would make more sense to go with a readily available and probably less expensive MH setup, where at least you can get some sort of information about those all important PAR values that can help you make good lighting choices.