Also a 10x Dev would usually go back to the product manager/client and say that feature X looks kind of redundant or a really edge case, but would take a significant chunk of time to code. Is it really needed for v1.0 considering it will cost 2 weeks of Dev Time?

You obviously need to have a good understanding of your clients and their needs and the way they use your product to e able to do that, but many shops simply ignore introducing their developers to the actual users of the product.