@Martin *******
talk to your immigration officer, remember the 30 day under consideration is the first month of your new extension (as a rule) so it's added to your current stamp.
You make sure that your old re-entry permit covers you leaving during that time period (confirm it with the immigration office)
I'd hate for you to be penny wise and pound foolish and lose the entire extension because you wouldn't spend 1000baht on a re-entry permit leaving during the under consideration time period.