phrasius in that thread reckons XBMC doesn't use the extra CPU power... Now, I haven't tried such an upgrade myself (doubt I'll ever get enough spare time/money/boredom), but I'm pretty sure that's just not true. At one point support was specifically built in, and FriendTech even donated a DreamX to Team XBMC so they could test it out.
Sure, I gather the benefits are slim, but they are at least existant.