Sure if you are not flipping switches then there is some performance boost.
I was thinking you meant adding a crazy number of switches to support every possible shader situation.
“I imagine it doesn’t matter much for very simple materials like above. But if the math nodes keep piling up, could the performance be increased by splitting it into separate materials to get rid of all the adds and multiplies?”
Only if there are things hooked up and compiled that are not making a visual difference for a good number of the materials. Ie, you could do a bunch of expensive operations and then multiply them by a scalarparameter with a value of 0 before adding to a material. That would “turn off” the effect without requiring a new shader permutation, but it would also always run those instructions for all instances, since materials do not branch based on masking by dynamic parameters.
In your above example, emissive with strength of 0 will cost something, however small. In that case its probably so slow that it is not worth worrying about. If your emissive operation was really expensive then I would be concerned.