I have a friend that did the conversion on his 96, which both his and mine are built very similar (4in,bumpers,35's, sliders, etc.). His isn't any faster while climbing hills nor does he get any noticeable mileage improvement (they get what they get). The only benefit that I can see is the front isn't turning. Slightly different feeling while driving.
Believe me, I was very against the full-time system on 80's and seriously considered converting mine (my LX when I first bought it). I came from a part-time 4wd world and never understood the reasoning behind the full-time system. After owning two of them, I plan to leave it just the way it is. It is very handy for changing weather, dirt roads, etc. Like most have said, no real improvement/benefit and the extensive posts on 'Mud bear this out. If you are planning on turning it into a trail only machine and are going to be running Longfields, then part-time away.
Putting in an Atlas opens a whole other can of worms (read modified to removal of gas tank, different/centered rear axle).
Jack