Looks to me you are doing fine regardless.. not too many can compete with RDM at low level range in terms of dps (due to the nature of dualcast), that is, if those numbers are even relevant for a synced low level dungeon.
And given that, your dps aren't even that much of a different.

Like some had mentioned, jobs aren't balanced around that level range, at least not RDM - they start at 50 for a reason, while the old fashion jobs at least have a progression of 1-49.

Sounds to me that you were just grouped with a whiner.. If I have to nitpick from this, even he wasn't doing the best "dps" he could do at that level, I ran enough sync'd runs on my RDM to know what numbers are like. That 20 or so dps isn't really going to make the clear speed that much faster, if any.

Wait till you are 70, geared and do more dps than most RDMs haha.