That’s a 33% increase, so it will definitely feel heavier if you are not used to that weight. As far as a goal weight, it would really depend on your goals.
My main reason for sandbag over shoulder is to help with throws/takedowns in my MMA/krav maga training. A human is much more cumbersome per pound than a sandbag; I have noticed that guys weighing around 200 pounds are no longer a problem for me to pick up and manhandle since I was consistently throwing a 250 lb sandbag around.
I have no experience with stones yet, but from my understanding, a sandbag is more difficult per pound than a stone. So if you’re using them to improve your stone work, then you should be comfortable with a sandbag that’s the same weight as the competition stone.
It’s annoying to add and remove sand from bags, so you can load some small bags with sand to add/remove weight. This will allow smaller jumps.