Donny artical is great info. And number one on your list, yes ,a mixer will give you more headroom by boosting the keyboards output. ANd the other benefits mentioned there. But you do have another piece to move and setup.
For myself one of the reasons I love the BK9 is that I don't need a mixer. As you know it has a mic input and AUX input for my IPad so all I need to do is plug the BK9 into my powered speakers. I've never put my master volume on the keyboard past 12 and never past 2 o'clock on my speakers. Plenty of headroom.
"Could you turn down, we want to talk" LOL