Sub-50 PvP is indicative of ....nothing in lev 50 PvP. The reality is that even in lev 50 PvP you can compensate for a lot of the Merc dps' shortcomings with raw player skill. Can a Merc dps be one of the 20 best PvP toons on a server? Yes, by having the absolute best player skill. But that is what is always comes down to. At the high skill levels, you MUST be a better player in order to achieve the same results with Merc dps. Because Merc dps is hands down the worst class in the game.
Im glad you worded it that way because there are some really good mercs out there, both arsenal and pyro. The thing about Merc is your not going to be superman on your own, you need to go with the larger groups and really find a good spot.
When i PVP on my Arsenal Merc i aim to get in the top 3 of DPS, we not the class to carry the Hutt ball so stay and control mid area, CW go to the mid gun and use pylons for LOS help, VS go with the larger force because Death from above is really needed to break up groups... the trick is and always has been play smart.. dont get trapped in a 1 v 1.. safty in numbers