I've done some testing on this, using the same 2 mono tracks that comprise the final stereo mix.
1. Panning Test: As described in the OP, when the tracks are panned hard L & R as intended, the meters show a peak in the red of +2.8Db, there is no audible distortion, and the waveform display shows peaks reaching -.2Db
When either track is panned to the Center, the meters show a peak of -.2Db, consistent with the visible waveform display. When the tracks are panned somewhere between, then the peak is likewise somewhere in between -.2Db and 2.8Db
Conclusion - Certainly seems to be the pan law at work here, giving a boost of 3Db to a track panned hard L or R.
2. Bouncing Test: I bounced the 2 panned mono tracks to a single stereo track. The new stereo track also has a peak in the red of +2.8Db, and no audible distortion, it sounds great.
The waveform display for the new track does show peaks hitting +2.8Db [As stated before, the mix is not a brick wall mix. And so the peaks are nearly all transients from kick and snare hits].
3. Export / Import Test: I exported the 2 tracks to a 192K mp3 (format specified by the Taxi website where I post some music). Then I re-imported the stereo mp3 back into Sonar and played it back. The peaks are exactly at 0 Db, and once again, it sounds great, no audible distortion.
The waveform display shows peaks hitting exactly 0Db.
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Bottom line ... This is not creating a problem per se, so no reason to change my Sonar pan law setting. But I would like to understand how these tracks can go nearly 3Db into the red without distorting, mainly so I might find a way to increase the levels of my own mixes to be more commercially competitive in the genres I produce (note ... I didn't start the loudness war, I don't like it, but it's a fact of life IMO, that isn't going away any time soon).
If no one has an answer, then at least this posting might help other users who run into similar conditions with their projects.
Thanks