performance chip or bolt on horse power ! or better gas mileage,on or the other

[smsrobertks]

99 yukon 350. i dont know how much hosre power it has but it does ok.i think it has a chip in it that limits it to 99 miles an hour or a rev limiter or both.can i change the chip to gain more horsepower. would a bigger thotlle body and or bigger injectors make more horse power,well i know it will but can anybody guide me what would or should i change and why. underdrive pulleys are they a horse power gain is there any gain there? if so wher is a good place to pick these parts up. i just live about 25 minutes from summit racing.any ideas or suggestions. if i cant get more horse power can i change the performance to get better gas mileage.i would like one or the other. this is not life or death issue but i am interested. thanks to all!