Adding tempo marking through shortcut does not alter the playback speed
Adding a tempo marking through the shortcut (shift + alt + T) does not alter the playback speed of the soundfont, at least for MuseSounds. It will alter the speed of the line that reads the notes, but not for the actual sound. This is only through the shortcut addition, and works fine using the menu drop-in marking.
Comments
What tempo are you adding? It works for me with one single exception: if you leave the tempo as 80 without editing it, then it gets ignored. Edit it (delete and retype the 80) to work around that.