Scribe

You have to not think in terms of "stages" as there is no hard and fast conventions for what it corresponds with. That term has stuck and is pretty much the least informative marketing speak. It's supposed to be a shortcut for indicating a difference between two products that are similar. You can't use it to compare across anything other than a single product line and it doesn't mean anything between manufacturers or even between two product lines of the same manufacturer.

Stage one for one company might be stage four for another. For example, as a manufacturer I can have a stage one coffee mug (guess what I'm drinking) and a stage two coffee mug. What does that tell you? Nothing. It could be the capacity, the thermal properties, or even the addition of a "World's Greatest Jackass" logo. Now assume another coffee mug maker says they have a stage seven coffee mug. What does that mean? Does it mean that it's better than my stage two mug? Does it have a missile launcher? Who knows. Okay, now say that I (still making coffee mugs) now make a stage one and two water bottle... still tells you nothing.

It's better to think of these things in terms of their actual properties. In the case of turbo kits, it's things like the CFM and spool properties of the turbochargers. Yeah, it requires being more informed on what all those numbers mean and how to read a compressor map, but it's the only way to really differentiate these things.

Oh and to answer your question, the exhaust has a huge part to do with their results. Look at the difference between the power achieved by any kit with the stock versus a full 3" exhaust. It's massive (usually the difference between 475-550whp setups) because turbos work off of a pressure differential.There are other things to consider as well, beyond just initial power numbers on a choked system with no boost controller.