"Is it worth it?" is more of an opinion than a fact. For me, some of my mods have definately been worth it (short shifter, rear sway bar). But then I also have plenty of mods that I wouldn't do again (CF hood, $1200 suspension). Each one of my performance mods did make a difference. The $$$ to HP ratio is a tough question to answer. Yeah, you could buy a turbo, but that increases your chance of blowing up your motor if you don't do it right. With most of the bolt on mods (intake, header, exhaust), the likelihood of blowing something up is low.
Bottom line is, is it worth it to you.