No, where the TJ is concerned, the K&N air filter adds no power. None whatsoever. A K&N can add power to some vehicles that have restrictive air intake systems, like the HP Mustang 5.0L or Camaro Z-28, but not the TJ since the TJ's air intake system is completely non-restrictive.
Neither will it improve your mpg.
What will a K&N air filter increase? The amount of dirt and fine particulates/silica/etc. that make it past the air filter and into the engine where they will cause additional engine wear.
For a Jeep, especially one that sees dusty offroad conditions, avoid K&N air filters like the plague.
A forum community dedicated to Jeep Wrangler owners and enthusiasts. Come join the discussion about reviews, performance, trail riding, gear, suspension, tires, classifieds, troubleshooting, maintenance, for all JL, JT, JK, TJ, YJ, and CJ models!