If I import an MP3 to an open audio track in a session, does Sonar do anything to the stereo imaging? Or will it be the same as if it was played through an MP3 player.

Depending on the kbs of the mp3 file which could be anywhere from very low quality ( 128kbs ) to medium quality,  ( 320 kbs)  Sonar converts the mp3 into a Wave 32 bit file. The quality does not get better, that's impossible,  but it doesn't make it worse either. Should be the identical pour to medium quality of the original.  

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Was there perhaps a difference in the pan laws between the programs? That would certainly affect stereo imaging.

128kbps is medium quality MP3 while 320kbs is considered to be as close to CD quality for an MP3.

VBR or CBR? what stereo setting was used when encoded? these can also effect how it's decoded

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maybe reaper treated it as 2x mono channels rather than a single joint-stereo, for example? still, interesting...

mudgel
Was there perhaps a difference in the pan laws between the programs? That would certainly affect stereo imaging.

128kbps is medium quality MP3 while 320kbs is considered to be as close to CD quality for an MP3.

 
Not to want to start a fist fight this early in the morning :) But I will beg to differ-- 128 is a very low quality audio format and more or less the bottom of the scale,  and 320 is where it becomes less noticable to most ears but far from CD quality. It's still has removed more than half the data. 
 
Interesting test Happy. I can see this as for sure there is a difference in the encoding, decoding software. I would never use an mp3 as part of a recording myself. To me that's like recording my guitar with dead strings and a $10 mike.