Well lets compare to cpu world. Intel chip for example. Ivy bridge to haswell. Extremely negligible gain, not really even worth the sunk cost of RD. From consumer perspective not even worth the upgrade. Any diff in the bitcoin world, i think not.
CPU performance scaling is limited by other factors than just process node, if you want to compare performance progression over node history take a look at GPUs since they are more similar to mining gear (can just add more cores for improved performance so much better scaling with node shrinks). Beside Ivy and Haswell are both on 22nm and Intel was mainly focusing on performance/w and GPU performance.