In the early release dates of the new M, someone posted that the factory, and or user programs did not make use of the side screen display, and that only exclusively new programs populated that handy dandy new screen.

Whoever posted that is misinformed.

The side screen displays content from ANY performance. It makes no different if it is a preset performance, a user performance or a performance in a library.

Essentially when you load previous content it is converted to a structure identical to new content - so there would be nothing limiting in loading older content even if it came from before the Montage Classic (like the Motif XS content which is allowed to be loaded).   Whatever limitations those previous generations had would be removed and available to the converted Performances.  

So even though previous generations allowed 8 elements maximum per "collection" (was called voice before, Part more recently) - that imported data could utilize (meaning add more to total) all 128 elements per Part.   And certainly the functionality of the second screen would remain fully functional for converted data.