I had the same idea with LivePrompter - and I actually implemented it. Not so much tempo, but rather transpose and some display settings (hide chords, show chords…). In the end, I didn’t use it as much as I expected.
Both with Cantabile and LivePrompter songs, I’ve now firmly separated my projects and have one song folder for each band; whenever I bring a song over from one to the other, I make a copy. My experience has been that song files will evolve over time, and the changes I make to fit the style or arrangement of one band will not necessarily fit the other.
So I wouldn’t really use this feature, but it could make sense e.g. for bands that regularly play with different singers, so creating a setlist for a gig with a specific singer would allow to keep the song files as they are and simply add transpose in the setlist. This does work with LivePrompter - and could be valuable in Cantabile as well - although using global transpose as a crutch is usually frowned upon by keyboardists (gotta be able to play all keys ) . I generally transpose the chords in LivePrompter and then play in the new key without adjusting Cantabile - keeps me agile…
But I admit; I’ve been guilty of using global transpose occasionnally for quick on-the-fly experiments in crazy keys at rehearsals…