I ended up trying assault. You should try pyro.
Go try pyro on a powertech and mercenary both, then tell me how you feel about that being the mercs best PVP option.
I'm dead serious because I agree that Pyro is probably the better tree for merc on PvP DPS. If you try it on both ACs, you will see the problem pretty easily. Whether you make it work just fine as merc or not.