Well, I haven't tried feeding Grey Catbirds in my area (the most I see at any one time is 2 birds). The chances of a catird coming to your feeder are the same as the chances of a Pileated Woodpecker coming to your feeder, which is pretty slim. Pileateds are pretty shy, and are not easy to attract to feeders. This explains why my father has seen one 3 times this year, and I still haven't seen it. The chances of attrating this woodpecker may increase somewhat in the winter.
Grey Catbirds, on the other hand, I have seen them thoughout the warmer months, and for good reason: abundance of it main foods, which are insects, other invertebrates, and berries. So unless you don't mind having live insects crawling around in your table feeder, I wouldn't expect any catbirds at the feeder. I do however, see the local catbirds on my lawn, although this occurs probably more often than I've seen (thrice I've seen them on my lawn). So don't give up hope. If you have berry plants growing around your property, maybe you could attract them by collecting some of the berries, and spreading them around a known catbird hotspot.
Maybe some other members have had more success than I have, and they'll reply soon.