I think he knows Lestrade's name, he just likes to irritate him. <--Perhaps, but I think he feels that "Greg" is unnecessary when "Lestrade" works just fine. He makes an effort to remember Tom because he cares about Molly.
He looks like a ten year old talking to his cat. Of course, with Sherlock, talking to the cat would soon evolve to dissecting the cat, which would subsequently be taken away from him, which would lead to a tantrum of epic proportions. :-)