Choices are pretty hard using Sonar. So much is packed in there. I think you will find as you go along Perfect Space fits certain cases while Breverb does others. I try to fit the tool to the project that I'm working on. The longer you do this you will begin to see your bread and butter list growing.
As far as acoustic jazz is concerned, the synth plugins are pretty good but you will probably want to upgrade to some real quality pianos. That is if you are using plugin synths. If you do a lot of electronic music, Rapture and Zeta will fill the bill nicely.
The ProChannel FX are the strongest move Cake has made in a long time. They are absolutely hands down at the top of the list of my bread and butter go tos. It is great to be able to build a custom channel strip for each instrument on every track. Once you get ProChannel down everything else will fall into place.
For mastering I use Ozone 5 but I would not blink an eye just throwing Concrete Limiter on the end. I love the CA2A compressor on tracks. The ProChannel EQ is kind of hard to come to grips with at first but once you understand how to use it it will blow Sonitus out of the water. All of the ProChannel plugs do a great job. But again, each one of the plugs has something it does better than the other in certain situations.
I moved up to X3 this past year and am enjoying all the new features. Somethings have disappeared in the new version but if you keep your old versions on the drive you can always scan those and use them.
I could go on but I tend to sound like a commercial for Cakewalk. I've been using Cake products in studios since about 97. Some versions have been exceptional and some have been calamities. I will say even at their worst I have always been able to deliver a quality product on time.
Since you already have been thru Sonar 7 you are already aware of the products strong points. Just roll up the sleeves and dive in. Try loading an older 7 project into X2 and start playing around. I WOULD advise loading just a midi project and learning how setup the plugin synths. Loading an older project with plugins that are no longer available to you could be confusing. I wish you luck.