a long while ago i had the pleasure of installing an avenger supercharger on a heavily modified jeep tj. it had the 4.0l and 6 spd. from what i remember of the install and the aftermath, id say it was a lot of money spent and moderate gains. avenger did not have a computer tune upgrade. and from what ive heard that jeep i installed the avenger on had a custome tune done on it and that helped. but again, it cost a lot. so if you want to go the supercharger route then be sure that it is a very complete kit with the computer tuning.
i did a turbo as well and that one was impressive at mid to upper rpms. it was beyond a typical jeep actually.
personally, id waste my money stroking and poking. more suited for the low rpm torque we really need;)