mudgel
There seems to be two camps here.
1) mastering = the finishing of the material via a number of plugins getting the levels and sound correct
2) mastering = the setting up of the material for release on external media, gaps between tracks and adding metadata.
Sonar does 1) really well with the tools that come with it. For 2) Sonar needs to be supplemented with external programs or have additional features added.
There are two proverbial camps, but realistically, and historically, they are both combined to make the Mastering process. Today, they're many times split-duties between multiple people. In both cases, the idea is to best match the audio quality to the medium it's destined for. If all you need is an .wav file, by exporting from Sonar with some excellent plug-ins to best suit your needs, this is Mastering to .wav. However, if you want to cut an LP, you will split the duties likely to make your audio pristine coming out of Sonar, then goes to your lacquer cutter, and finally the replication plant. The audio finalization processing from Sonar to a transmitted medium, such as a CD-R is the first stage, but then the lacquer-cutting and even as far as making the mother plates can all be considered a collective effort in the Mastering process.
I do believe that Mastering has, in some people's minds become just a finalization process from the DAW to any type of medium. Technically, as long as the destination medium is met, such as burning a CD-R for CD replication or .mp3 upload, then Mastering was completed in Sonar. So, it can be called a Mastering tool in that regard. That falls into camp 1. But for practically any other medium additional Mastering work must be done, and camp 2 is added, but doesn't replace camp 1, IMO.