This is my experience too. Currently am using K&N Stock filter for my Alto. Have used it for more than 10k kms now. Only thing is engine breathes easier. No significant improvement in performance. In fact I have also used K&N Performance filter as well (the conical one). Here there will be an increase in noise and marginal improvement in performance. As per my knowledge, to get significant boost to performance and improve low end torque, Performance filter has to be supported by FFE.
Good that Surfer is able to feel the difference