For sustained pve dps only, grab a couple of similarly geared 50s who know their rotations and test them against a low-level world boss (for example the robot on Nar Shaddaa), you can get about 4 minutes of sustained dps in and then reset. Based on a few attempts at parsing I would expect a sniper to be substantially behind a mercenary (and a sorcerer for that matter, among ranged dps) by about 20-30%. But don't take my word for it, try it yourself and maybe you can do better; just don't be satisfied with handwaving analysis based on one or two skills or saying "there is no way to know until combat logs".