Officially, you can only "setInitialText:
" and can not retrieve the text when the user is done entering it.
If the text were available as a property, then yes you could make use of it when the completion handler is called; but Apple has chosen not to make it available and therefore it's not currently possible through the official API's.
If this is something you need, I'd recommend filing a bug / enhancement request with Apple to ask them to consider it.
Now, all that said, I see that SLComposeViewController does inherit from UIViewController which means you can look at all the subviews of the view owned by the view controller. If you enumerate the subviews, look for the ones that are UITextViews or UITextField and there's where you should be able to find your text.