Well Sid1962, did you get your answer?
We've enjoyed many specialty dinners onboard =X= and do not always tip more. The $30 does cover the gratuity, period.
Like other posters mention, if you wish to give an additional gratuity, for the service, etc., then feel free.
We were on board Princess last month and did not tip additional in their specialty restaurants (Sabatinis and the Steak House); we just didn't feel the service, food and presentation warranted anything additional, IMHO. On board =X= we realize we are not expected to tip more than the $30 surcharge, but usually will add another 20 on the tab, as their service and presentation is usually excellent.
Like another poster mentioned, often a meal such as what Oceanliners (or the other restaurants) prepares and serves, easily costs $200, at a restaurant here in SF.