I've got a Mercenary and also a Gunslinger, and on both toons hate the Middle DPS trees due to lack of maneuverability (sic?), and much prefer the right hand side tree (The PvP tree), due to the greater movement allowed. Are these 'PvP' builds viable in Hard Mode Flashpoints and Ops?
What makes you think the "right hand tree" is the PVP tree? I run a Lethality sniper in end-game ops - the mirror of the Dirty Fighting gunslinger - and it's not only viable, it's got the edge on Marksmanship (the equivalent of Sharpshooter) in my opinion.
You might distribute your points slightly differently within the tree for a PVE rather than a PVP build - as an example, a PVE slinger has no use for Flash Powder, Dirty Trickster and Dirty Escape whereas in PVP they're key survivability skills - but the right 5/5/31 build will make for super effective single target damage on operations bosses.