Not exactly the answer you’re looking for, but the plugins I can’t live without for mastering are the Waves stuff. Which, of course, doesn’t come bundled with Sonar.
In particular, their limiter - the L1 or L2 - and their multiband compressor - C4 or C6.
I see they have a Mastering bundle on sale now for $139, which includes the L2 and what they call the linear phase multiband compressor (which looks like the C4 to me), plus a linear phase eq (ok, but nothing that special). Worth every penny if you ask me, if you plan to do your own mastering.
Edit: btw, I would use these on a stereo track of your final mix. Wouldn’t advise trying to use them directly within your mix file. IOW, export your mix to a 2-track stereo file, 32 bits, and then reimport that to Sonar. Use the L2 on the output bus to raise the volume and drop to 16 bits. The others to color and season the mix to your liking.