This depends on your search engine positioning, if you are happy with it then I'd say don't bother.
The benefit of a Yahoo listing is diminishing and a site that has 20,000 members could probably do much better with a competition.

Why not offer a $300 prize draw which only needs a member to invite a friend to get in the draw, run it for a month or two before drawing it and hey presto, many new members!

Repeat at will