I usually use 120 in my curved shaper. 150 is a little fine but works ok.
Anything under 120 can be a little too gritty for my liking. Going down to 100 grit for instance.
120 will shape and scuff nicely. More grit and you may be taking off more leather than really needed.
Just my humble opinion. Everyone has their own taste.
You can use 80 grit discs on an angle grinder if one wishes to.
Get the job done, lose the least amount of material.
I mean, a person wouldn't use 80 grit when refinishing a nice piece of furniture, would they. You would use the finest grit of paper that still allows for a decent job.
Bruce, 220 and 320 is for sanding/shaping shafts in the beginning stages, or any other piece of wood for that matter, when you are going to see some saw dust flying around.
Why would you feel the need to touch a leather tip to one of those grits. Why would a person use 220 when 120 will do the job just as well.
Using salt and pepper to spice up your dinner when cooking doesn't mean that using more is better. You know, if a little salt is going to make it taste good, more salt will make it taste better. Doesn't work that way. Same with paper.