I just added XM radio to the stock 6 disc changer by using the cassette tape provided in the XM Roady. I had the XM Commander in my last set up, Mercury Marauder with after market Alpine system, and the guy who installed that system told me that utilizing the cassette tape heads was a much better alternative than using the FM Modulator, second only to direct connect which may or may not be possible with the stock player.
I am happy with the quality of sound so far. I have the updated speakers and subwoofer/amplifier in my system. XM sounds better than CD's in this current system. I must say that it is not horrible for a stock system overall. If I were you I would purchase the XM Roady for $50 less than the XM Commander and install the cassette tape option yourself.