Metering is certainly one thing to have for Mastering here, but workflow is even more important, IMO. Personally, I'd like to see a Screenset dedicated to [M] for Mastering, and the 'console' workflow could be set up just like a real mastering desk. If it were, say, one large stereo channel with an input, output, and all the plug-in slots in between (where you can see all the plugins at once (scrollable view), the workflow would be worth good use. Wavelab has an excellent workflow, where you see your stereo track, and one plug-in chain. On top, you can choose metering options. But the first and foremost need, IMO is workflow. Metering is part of it, but cumbersome if the GUI isn't made to allow metering to be present at all times.
Yes, there are Mastering templates, and yes, you
can Master in Sonar as-is, but it's the last place (currently) that I would choose because it doesn't have a Mastering workflow. But yes, metering included would be divine. :)