I could not find any conclusive info while searching AWS docs or googling. My guess is that no, origin shield would not be necessary, if the origin is just an S3 bucket? Or, is there some benefit I'm missing? Thanks!
One of the benefits of using shielding is that it can reduce the origin load, but as you're using S3, it really doesn't matter.
Better network performance and cache hit, in this case depends, because as you need to pay for the extra new shielding layer, the "better things" are already done by AWS under the hood and it's probably more expensive if you "enable" it, depends if your shielding is in the same region, the users too...
Also, you can't see how good is the shielding, up today, is impossible to get a graph with that information, you only have the Cache Statistics graphs. And the billing account is complex.
If you can't monitor clear the app behind CF and see the advantage/disavantage, don't use it.