To add a bit to the conversation,

Engine modifications will always enherintly modify the power band in some way, however with your basic upgrades you will really only free up additional horsepower that was already there. To really adjust where you want the power on this kind of an engine, your going to have to modify the air flow. remember spark, and fuel are basiclly backups to the real control of power, AIR. so if you change the way the air gets to the engine you change the power. For instance JCW's use a bigger compressor housing to take full advantage of the higher boost levels. Whats cool is that by changing the housings size your not so much changing how much air it can push rather where it does it best. this is becuase the turbo will be spooling slightly later in the game so all of that neck snaping power you get in the begining gets bumped a couple of revs north.....cool huh?