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.
Buddy, spending 5K on K&N I don't think is wise. I had used it in my Honda City for about 2 years and the difference I felt with K&N in comparison to normal filter was marginal only in regards to drive and acceleration. Apart from filter you'll need a cleaning kit too which comes around 1200-1500 rupees. Though I did not use the kit with my car's K&N Filter but it is advisable to use it. Now I have reverted back to OEM filter only in my car as the drive with K&N felt boring and atleast every 10K kms I can simply replace it with new one for 400-500 rupees.