Indeed a system text is a usefull way not to have to re-write anything in the parts but if you happen to have a tempo mark exactly at the same position there is still one caveat at the moment.
No matter what you do a tempo mark will be always placed above a system text.
There is no way to change this vertical hierarchy at the moment.
If you want to use a system text as a kind of subtitle you probably want to have it above a tempo mark.
Of course you can manually move the tempo mark but you will have to move it for each part because as we know changes made in properties in the score do not apply to the parts.