you got a lot of mods there and i am new to the xterra but if it is like any other engine these things will help. Since you have forced induction you should look to porting some items. Throttle body, intake manifold runners, and take the head off and send it out a machine shop and improve the airflow. I come from hondas where this is a step that can get pretty decent gains from a motor. If you can find someone to cut you some cams with a more aggressive profile but you don't want much since you have F/I. Check a crower cams they may be able to help you. Also depending on budget someone can make you a set of headers so they don't involve removing cats (damn expensive though) I am pretty sure S/C engine prefer short runners and long downpipe if you go for it. Chips aren't the best product since they aren't programmed to your specfic engine and mods unless you are able to have them programmed but that deals with dyno tuning. Fuel is another thing to look at. Upgrade injectors RC injectors can balance and blueprint yours i am pretty sure and if you can get a fuel pressure regulator and modifiy it to fit the X that would help but again when you really start to get serious into horsepower you have to get on a dyno and start messing with fuel, ignition and timing to find the best power b/c if you don't you could be actually losing power. Good Luck in you adventures to find more power. Hey if a 1.8L Honda motor can put out 500+ hp the X's 3.3L has power you just have to find it. I am sure the block of 3.3 can handle a ton of power, Nissan makes strong motors. Hope this helped you out!