I think its only the stuff that goes into the Cakewallk Content folder like whatever this releases demo song is, tutorial files and such. That's based on past experience as I'm still downloading. There shouldn't be anything critical in the extra content that would stop SONAR X2 running.
Plugins (apart from Dim Pro and Rapture) are part of the main installation program. ie parts 1 to 4 of the installer.