If this is an impreza as you've stated, and not a turbo, there is nothing you can do to increase power by a substantial or even noticeable amount.
If you want to do intake and headers for the noise? Go for it.
Just don't go expecting 50hp from them.
Your headers are equal length. If you want to go unequal, you may have a hard time finding UEL NA headers.
Why the upgraded clutch?
Any increase shock your transmission will see because of that will be transmitted down the entire drivetrain.