Composing in P5, and then mastering audio in Sonar

This board seems pretty dead, but here it goes nonetheless. Project5 is simply great...if FL Studio is so popular, why isn't P5? They're basically the same thing, but P5 is easier and more efficient. But I digress.

I am quite experienced at composing music in P5v2, but I am not at all familiar with the process of "mastering audio." I think doing so would make my music sound better, so I got Sonar X1. I would like to know how I can take a composition from P5 and master it in X1. This branches into a few sub-questions.

My songs consist entirely of MIDI played through VST soft synths - my favorites are z3ta, Nexus, and Garritan Orchestra. Imagine a finished composition in P5, ready for mastering. Should I mix down each track individually in P5 and then re-mix all the tracks in Sonar? Or should I mix down the whole composition in P5 and then apply some effects in Sonar? The former seems more thorough, but how would that be any better than adjusting the volume levels in P5 prior to mixing down? I know it doesn't have a mixer board, but a mixer board is just a glorified set of volume sliders. 

If there is some other way of doing this not mentioned above, please blow my mind.