The only reason anyone ever notices an increase in power or mpg on a Wrangler TJ after installing a K&N is due to the well-known Placebo Effect.
Yes, a K&N can help improve performance with SOME vehicles that have a restrictive air intake system. However, the TJ's air intake was specifically (!) designed from day-one to be completely non-restrictive so a K&N won't help with a Wrangler TJ in the least. The TJ's engine is not starving for air at all, the factory air intake and paper element air filter can easily flow far more air than the engine can ask for even at wide-open throttle and redline rpms. Really.
In fact, all your engine will get more of with a K&N is dirt. Really.