"As of yet" means the same thing as "as of now"; however, "as of yet" is usually used in the negative sense. "As of yet, he hasn't started." "As of yet, we've received no information." "As of now" can be used both in the positive and negative sense: "As of now, we have completed 50% of the project." "As of now, we have 134 members." It should also be noted that "as of yet" would normally be used in more formal writing and speech. In every day speech, you would more likely hear just "yet": "He hasn't started yet."

No, it isn't. He hasn't been seen, as of yet. We have no plans, as of now. (We haven't made any plans, as of yet.) Technically, they both refer to the present, but I think you see the difference.

I consider these expressions to be equivalent.