As Matt said, sharing languages is recommended. Also, you can swap a Document in and out of a SyntaxEditor. So you could have a single SyntaxEditor instance and simply retain the Documents that are open and swap them in and out as the tab changes.
With the stuff we're doing for v4.0, Documents will use less memory overall. Also you'll be able to turn off features like lexical parsing, outlining, etc., which will remove all memory used by those features. That will be very useful for situations where a document is swapped out and you don't want it using much memory other than the text part of the document.