I was 39.5 in the old orange Instinct slippers (lots of pairs) but wore 40 in the lace. Now I wear 40 for everything, Lace, Velcro and new SR Slipper.
I like the new Instinct SR but the blue strip on the heel isn’t good when heel hooking but if you’re climbing in serious heel hooking territory then you probably won’t be wearing slippers anyway.