all songs from P5 should open in Sonar or MC6 as long as they're all .cwp or .cwb files (or possibly the old .bun files - but those should also open (not import) into Sonar or MC, but might have more difficulty).
if they're not saved as .cwp, .cwb or .bun then they won't open in Sonar OR MC. and whatever doesn't work in Sonar won't work in MC and visa versa because MC is just a stripped down version of Sonar.
the problem you'll see for opening those files in Sonar or MC from P5 files will be if you have P5 plugins that no longer exist on the machine you're on so that you'll get an error that it can't find that plugin when you open in Sonar or MC.