I have a level 33 Shadow, Infiltration spec. And the amount of damage I do is pretty dang good. Every time I do a warzone, I am near the top in damage done. When I'm in a group for a flashpoint or heroic quest, I usually have to slow down my dps because I pull off of whoever is tanking. I do get caught sometimes, but not often when in stealth, usually by the mobs that pace back in forth in a spot and turn around just as i move up. For moving through mobs however, use Mind Maze on the one on the far end and you can run right through him. I haven't tried a Sage yet or shadow tanking, so can't really vouch for them. However, I am leveling my dps shadow a lot faster than my Immortal Jugg, and I'm having a blast playing it. It seems like you should look at some guides or get some help from other players, or just don't bother playing a shadow.