And how much of a benefit it gives vs requiring to make a completely different set of shaders which you can't even reuse? That's a strong claim that you can't have good performance, after SPIR-V is translated by the compiler.
I am not sure what are you talking about here (how is setting up inputs for a shader related to SPIR-V, which is IR format for shader code?) but, as I said and you denied, this how you get a game running on a $200-$300 console look the same as a game running on a $1000-$2000 PC.