What is the highest weapon value of a gun in DFRPG? Because that's roughly what you're going to get with a small, bullet-sized item, regardless of the force, IMO. Although, I suppose meteors can do quite a bit of damage and they can be rather small.
Things mechanically propelled by gunpowder are a different story.
Put it this way: A knife is Weapon:1. A knife used by something with Mythic Strength is Weapon:7, despite still being the same small, knife-sized item. If that being with Mythic Strength throws ball bearings, are you still going to keep it at Weapon:4?
Also, it bears repeating that the Weapon rating of something doesn't necessarily correspond directly and literally to how much damage it can do. It's a representation of its general deadliness, and in this case, a Weapon:9 shot with ball-bearings might mean they're thrown not only at Mach 2, but with an incredible accuracy only possible with magic.
You can have Supernatural strength and punch people using your fists skill and do lots of damage, just like a wizard can hit people with fire using their discipline skill. If you have Supernatural Strength and you want to throw a rock at someone, it has to be on the scene. It has nothing to do with having a power. It has everything with having the 'stuff' to throw. Yes, I paid for Supernatural Strength but it doesn't mean there's something handy for me to throw at someone.
Having something specific to throw is one thing; but there can be and are settings where you don't have to declare that there's some loose object to throw because there's obviously going to be some loose object to throw. Barren plains devoid of any loose objects are pretty rare.
Besides, what I was talking about was less declaring that there's something to throw, and more about invoking that it would bypass the immunity. I was arguing that if a Wizard uses a force evocation to hurl a desk at something otherwise immune to magic, he shouldn't need to invoke further than that to get past that immunity.