Im using drum replacer on a mono track set to stereo interweaving.  It doesn't appear that the pan controls do anything other than fade in and out as a volume control would.  Id like to use the same instance on an overhead track to beef up a tom performance, and it would be much easier if I could just trigger the tom samples from the same instance and pan them.  

With stereo interleave PAN is a balance control. If you want a true PAN you must use mono.

Im afraid that doesnt make any sense to me?  Judging from this https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Region_FX.06.html    the pan is suppoised to act.. well... like a pan should
 
 In either stereo or interleaved mono - the pan behaves this way, it bleeds into the center only as I fade the pan more to the left?  This particular track is headed towards a stereo aux track with the rest of the kit.

For the Pan in DR to work properly, the track must be stereo interleave and panned center. If the stereo track is panned to one side and you pan the DR in the opposite direction it will fade out. If the track is set to mono then the pan in DR does nothing.

Perhaps the routing issue is with the Aux track? Have you tried a separate bus?