In my opinion, having played most of the multiplayer halo games, the DMR is indeed overpowered compaired to other weapons, the only other weapon that can drop a player faster at long range is a sniper, binary cannon, or the spartan laser. The DMR, as is, allows players to essentally spawn in with a sniper rifle. This is hardly fair to the other team. While it may be overpowered at closer ranges, a quick melee and two shots from the DMR will still bring down nearly anyone.
The only way to be save from the DMR is to either stay inside (which almost no maps have adequate indoor sections to facilitate staying inside as an effective tactic.) Or to counter with either a long range power weapon (Sniper, rail gun, spartan laser, binary cannon) but even then, the DMR still has a faster fire rate than all of those weapons, and nearly as much power as a sniper rifle.
So yes, the DMR needs a heavy damage drop along with perhaps even a clip size reduction, it could be considered a power weapon as it currently stands.