I agree with Erik, I think color is of higher importance for a display rather than growth. Mixed temp bulbs could probably be used to achieve both but there are trade offs. I've read Sanjay's articles and he doesn't speak too fondly of the XM 20K's. However, I can say from first hand experience that the corals color up nicely. I would say growth has been decent. Obviously nothing like a 10K, but everything I have has been developing nicely. Although I am not sure what my PAR levels are, my guess is they are greatly improved by my reflectors (LBs). I ultimately chose the XM's due to their cost. Since all 20k's drift spectrum sooner and should be replaced more often, I figured it would save in the long run.
Sean to answer your question, although higher PAR does translate to higher growth rates, there is a point at which too much PAR or photo saturation can occur. At that point, the zooanthellae shut down photosynthesis and growth stops. Also, burning can occur. In the example you listed, the 400W although higher in PAR, would cause photosaturation quickly and ultimately would likely result in damage, especially if the photoperiod was longer than a couple hours. Consequently the 175W would probably achieve better growth as the corals would be photosynthesizing all day. Granted, all this is assuming the nutrients are available to promote growth.