Hmmmm....let's see.
The S/C uses premium gas which is at $2/gal now.
From what I've heard most boost isn't apparent until 4k or over (I usually never go over 3.5 or 4k when driving). It's a heavy truck, so I just poke along and go for getting better gas mileage in the city.
The S/C owners "sound" like they're having more problems or quirks, including the engine shutting down on highways (part of their recall I believe).
The S/C is an add-on to the old engine, not part of a integrated design, so maybe that's why it's impact is limited.
Nissan moved the e-brake to the floor for the 2003, so if you offroad you're screwed as far as using this for a driving technique.
The Xterra clutch is a weak link in the system. Many of us that offroad are waiting for lower transfer case gears to come along so we don't have to go through clutches prematurely. However I still would rather have the standard than the auto.
As another option, you could buy a 2000 or 2001 and put in some of the engine performance mods that are slowly coming out. But it wil still be a heavy truck that just gets heavier as you put the offroading gear on it (ARB, skids, rear tire carrier, equipment, etc).
So where am I going with this? It is a good truck (actually a great truck for its utility and performance/price ratio). 3 of our clubmembers spent the weekend doing as much or more than the Disco we were travelling with could do. I'm just not sure how good a truck it is to spend trying to crank HP out of. It's got good torque, and if lower t-case gears come out, it will have more climbing ability than the truck can probably handle design-wise.
Check out
www.spencerlowracing.com They've got some performance mods that you could look through.
Also hanging out with MAXC will give you a lot of info on what mods they've done.