You've got it right, CAI, cat-back, headers, etc. Literally things that you bolt on to the motor. These parts are cheap compared to other more extensive internal modifications, forced induction, etc. The point the writer was trying to get across was how well the motor responds to mods, but I don't know if you could get 50 horsepower from bolt-ons alone (unless you consider cams and ECU bolt ons).