No XM experience here, but from pictures I've seen, the XM 20,000K 175W bulb may be a little dim at this wattage; I know Neowind here on the boards runs the 20,000K at 175W.
I just replaced my 175W bulbs this past week with 15,000K Coralvues which I am very pleased with so far. They took the place of a Ushio 10,000K and an AB 10,000K. So far, the Coralvues are just as intense as the 10,000Ks with a nice balance of blue and white in my opinion, and at $50 a pop, cheap too! I don't have any pics yet, but will post them later this week, after the bulbs are totally burned in.