Stage Levels (stage 1, stage 2, etc.) are used to differentiate the different 'levels' of performance gains.
Each higher stage level will have slightly more components in the kit and will produce a proportionate increase in performance. Each higher stage kit will increase in 'difficulty' of installation.
Stage 1: A slight but noticeable gain in performance and 'typically' can be installed by a person with average technical skills and basic tools.
Stage 3: A significant gain in performance but requires some in depth automotive mechanical knowledge for installation, and some specialized tools. We usually recommend professional installation for these kits.
Stage 4 & 5: These kits are all about improving the flow into and out of the engine. Increased flow thru the oversized Turbocharger into the engine creates stronger combustion. The oversized Intercooler lowers the intake charge temperature for a more dense intake gases resulting in a more powerful combustion. Hi Flow Intake, Intake Plumbing, oversized downpipe, and Performance Exhaust completes this kit. All this combines for increased power to the wheels!
These stage level definitions apply to engine, drivetrain, suspension, and brake upgrades. As with any performance improvements, the initial gains are relatively cost effective and noticeable. As you get further up the performance curve, the cost and difficulty per gains increases exponentially....