Maybe I just do a lot of Mob hunting, but those rates sound low.
I began a new habit of only wearing armor dropped from mobs and I'm nearly always decked-out in a full suit. I even have a few items in storage. I play exclusively on Hard; I don't know if that matters.
I've yet to build a decent MOB grinder in my new world, but from the dozens of run-ins I've had with witches I've yet to collect an actual potion from one. Also, have only receive a grand total of 6 empty bottles. I had thought those would be more common. Mostly get sticks and Redstone, occasionally gunpower and sugar.