They higher the stage, the more hp it's designed for.
So a Stage II is set up to handle more hp than a Stage I.
A good aftermarket intake should probably let you gain
anywhere from a 1-7 hp. Depends on the setup. What do
you mean by 'the exhaust'? If you just replace the cat-back,
there usually isn't a lot of gain, maybe 2-5. If you replace everything from the header's back, you should gain anywhere from 5-20. Again, depends on the setup. And these numbers do change depending on if you are talking about an N/A or a TT.
The TT numbers are usually higher. Hope this helps-
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