Basicly, the mitsu turbo becomes a bigger restriction than our crappy 8v heads at anything much better than stock levels.
Adding larger exhaust to a car equiped with a mitsu turbo may or may not cause your car to "boost creep" which means the wastegate can't stop the turbo from building more boost, and after a second or two it'll be like hitting a brick wall when the computer cuts fuel. You *should* be ok with 2.5 inch exhaust but don't go any larger.
Producing much more than 13 psi boost on a mitsu turbo puts the turbo way out of its effective RPM, causing a good percentage of power gained by the extra boost to be negated by the skyrocketed charge air temperature. (This holds true even on an intercooled vehicle)
The mitsu is fine as long as its working, but if you ever find yourself needing to replace it (or in a position to easily replace it, such as an engine rebuild), just upgrade to the garrett, you'll love it
My first upgrade list includes 4 items, they go too much hand in hand to leave one out.
1: Instrumentation, properly working boost guage that doesnt max out at your desired boost level(not a stocker), a/f ratio and EGT preferably. Oil pressure doesnt hurt either, because sooner or later the sending unit for the stocker is gonna die and freak you out. All the go fast in the world doesnt amount to anything if you smoke your engine.
2: Intercooler, stock system works pretty darn good and is easy to do. Try for an Indy or Super 60 type, they fit in the stock location but work a little better. An intercooler is more likely to save you from a broken/melted piston than any turbo trouble. Be sure to buy the loudest f'in BOV you can afford, and laugh when people try to figure out where the heck that noise is coming from
.
3: G-valve or some form of boost control. The ECU limits boost down low to "save" your transmission. This really wakes up a garrett equiped vehicle, and wont break your tranny unless its already on its way there.
4: Exhaust, the more you open it up (keeping in mind what I said above), the faster your turbo will spool.
Of course, don't even bother to upgrade anything if the car needs brake or suspension work. You'd be suprised how many SRT-4s I see rolling around with everything we offer out of the Mopar Performance book, with the brake warning sensors howling.
Nice body work, its a far cry from the way it was in the first page
Donovan's Dodge Garage has a wealth of information on these cars and performance upgrades.
TurboDodge.com 
I've read somewhere in here that garret is bigger and provide more boost, so they want more power, actually i want more power too but in here you cant drive fast. I planned to put the Borla exhaust first, then when i got more money i was gonna buy those 2.5" catbacks from FWD. But still not sure to work on the exhaust system first or engine upgrade. Could you recommend me the first 3 upgrade you would do to your engine, except K&N drop in, already got that in. Just wanna have an idea. It has just under 200,000 KM on it, so not sure want to rebuild the engine yet, although someone said my head gasket is almost gone. Thanks
