Hah I was about to link you to the thread I started on the same subject but @LaPaloude beat me to it!
I can actually add a little bit extra info now as in the past week I picked up a smaller SUP to finally find out if I could feel a difference at all.
I went from a 7’10 x 19.5 @110L board to a 6’10 x 20 @95L board so it’s not apples to apples with the volume but I can report back even at these early stages of getting used to a new board, it surfs and pumps much better!
I have definitely lost a bit of paddle speed so not sure how it would go in really small conditions but in the waves we had over the weekend I was getting 35 decent waves a session so it’s still a wave catching machine.
I don’t think I could go all the way down to 5’ though but can say I definitely noticed the difference of 1ft shorter and 15L.