I agree with Michael that sometimes cats feel like they're planing. My cat has deep V hull--at least as deep as your g-cat, but symmetric. No way that could plane, right? Well, when flying a hull high in brisk wind, though, it sometimes feels like the side of the hull starts planing. Hmm. Turn a V on its side and you've got a broad surface to potentially plane on. Some of the lightweight boats with broader transoms may well and truly plane. I've seen photos of the Taipans that really make you wonder--not enough hull left in the water to really be in displacement mode. This is all very speculative. If your friends are like mine, they only want proven, studied facts. That's where you need the source Michael suggested.

Frank Bethwaite has laid down the "dogma" and explicitly states several times that cats sail only in displacement mode. To prove your point, refer your monohull friends to his book HIGH PERFORMANCE SAILING, chapter 20, especially sections 20.7 through 20.10. He lays down the super-narrow hull theory with low drag, etc. To quote:

"Slender-hulled craft such as catamarans sail always in one mode--displacement sailing."