This can't be more wrong. To be honest, I'm surprised to find out that so many priests are still underestimating shadow. If you are finding it hard to get to top half on most of the boss fights in ICC, and top 5 in some, then you are probably doing something wrong or not maximizing your potential (or the very rare chance that you are in a guild w/ extremely high dps, higher than most of the guilds in world top 10, and all your non-boss mobs died in less than 6s).If you're beating anyone except Shaman, Moonkin, Arms Warriors, BM Hunters, Destro Locks, etc... then there's a very good chance that they're bad.
It's may be difficult for SP to beat "pure dps" in a stand-still, tank-n-spank fight, but fortunately, there aren't many encounters of that type in ICC. The more movement required / multi-doting & searing potential, the higher you will find yourself in damage done chart.