Back during Vanilla and TBC, rogues were very competitive in DPS in sub level cap instances. Now, I have no idea how any rogue could hope to keep up with what my frost mage was doing as I levelled it. I think in Mara, I was pulling 1800 dps on trash pulls, because of Blizzard. Tank threat has been upped so much that any competent tank at the right level range can keep mobs glued to him well enough that the dps can just AoE like mad. Until you hit 80, you can't really AoE on a rogue, so you will be at a disadvantage. That said, when you get your thrash blade (if you're combat), your dps gap should narrow significantly.