Just My 2 Cents & It Worked Great 4 Me.
(I Have a 96 XJ 2 Dr 4X4 w/ Hi Out 4.0 & AW4 Auto)

All I Needed to Get Extra Performance was: Mallory High Output Coil & 8mm Silicone Plug Wires / Ford MotoSport 4 Hole Injectors & 2.5" Turbo CAT w/ 2" Turbo Muffler & Exhaust Pipe. Plus a Real Cold Air Intake (Thru the Grill).

Throttle Response Is Significantly Better! (Especially from 35mph to 55mph)
At 35 it Takes Off like an Early 70's Small Block V-8! Great Passing Power at 55-65mph Too!
Mileage went from 14 to 16mpg in town & went Up from 17 to 18mpg Highway!
Not Great Mileage Gains. But, The Performance Gains Are Well Worth the Money!

Just Wish I could Afford the Time & Money 4 a Supercharger Set Up!
Now IMHO : That's The Way to Get Real Power Out of the Jeep 4.0 (Easy 300HP & 300 Ft Lbs Torque/ SWEET!)