No, rewire syncs the two softwares - P5 plays w/in SONAR. My (and other's) technique was to compose in P5 rewired to SONAR and then import the audio for further maipulation. You could archive your synths but have them readily available to edit. Freezing a synth means to turn it into audio so it doesn't drain your cpu. Not such a big thing anymore, but think of it as bouncing a clip down to audio w/ alll your effects printed. And it is simple to unfreeze it - simple click of the old button. P5 introduced the feature to Cakewalk DAWs, I believe.
I didn't talk about composing in SONAR except to say I do - it has many of the features of P5, plus a better render engine. That is good, if you care about the quality of your sound. One of the reasons I switched primarily to SONAR. I do agree that P5 is a great workflow, tho, expecially for loop based pattern music. However, many people do record audio. Sorry if my attempt to help offended you.
Mixing is more than adjusting your volumes. Many times I switch over to Sound Forge. It is easy in SONAR to open and send audio to SF and import it back. Sonar has a greater array of effects, which you have to buy separate in P5. More sends. The list goes on.
Anybody who peeks in at this forum has probably been using P5 since before it was dropped by Cakewalk. While it is great to see new people get excited, it is a little cheeky to get lectured about how great it is by someone who doesn't know basics like rewire and freezing.
Good luck and have fun.