Are cats competitive DPS?

Yes. The design is for cats to do slightly less DPS than rogues and hunters, since they cannot respec to tank or heal, however the gap is supposed to be only a few percent. This is close enough that gear and skill are usually the determining factors on the DPS meters, rather than class design - excepting overpowered and/or bugged specs. Cats are proving to be quite competitive in 3.1, with an average DPS on their most favorable fights currently within 10% of that of rogues.