hp=(tq X rpm)/5252
end of story. Most bolt ons just shift the power band.
Build the block, go higher compression, or boost it... Otherwise you're making minimal gains for a chunk of money.
Figure you can spend 300-400 on a nice header, and over 100 for a nice intake. Right there, you're half way to a turbo setup... I love watching people buy all these bolt ons, sell them for half what they spent, and then go turbo. Save, do it right the first time, and be happy!