Maybe it's all relative but I got a hell of a lot of use and abuse out of a 120hp Trooper from that era, and the 2.6L Montero. Later owning a 200+hp Blazer and 270+hp Jeep I have a ton more power, the Blazer was properly quick but I have to admit that the Jeep I drive now drives sluggishly compared to the much, much lighter 1991 Trooper of days past. It feels powerful up hills and it can pull out in front of a lot of things at a stoplight without much skinny pedal but it doesn't actually feel quick.
More power is a good thing, I'm not trying to talk you out of that - but I do think there's a lot to be said for thinking
lagom and making the best use of what you have before undertaking huge effort in making it more complicated.
Simple things first, I think your 12v engine was originally supposed to make about 140 horsepower and 170lbs torque... do you think it's actually producing those numbers? How's your fuel filter, have you replaced it? The injectors on the 6g72 don't have a ton of reserve bandwidth to begin with and anything that affects their pressure will have a huge impact on fuel delivery - subsequently horsepower AND MPG's (gasoline releases more energy per pound when rich, than when lean... so often a lean engine will get fewer MPG's than a rich one - while also delivering less power)
I haven't owned a 3.0 Montero, but I've been under a couple.. that exhaust system doesn't look sexy. I'd bet a smoother (not necessarily louder, just smoother/wider) exhaust would uncork enough extra pokemans to make you feel it every mile of your drive.
I wouldn't play with Dust-O-Matic open element intakes, not worth the damage if they don't improve filtration. There's probably some room for improvement in the bore of the throttle body (or maybe a swap over from a 3.3 or 3.8?) and the general porting/cleaning of the intake manifold.. I kinda go crazy cleaning any used car's engine when I get it.. inside/out.. amazing how many small problems magically reveal themselves that way and in aggregate how much the vehicle will wake up when they're resolved. <img src="/forums/images/graemlins/kewl.gif" alt="" />