No worries.
In all honesty, the easiest answer to your question of "Most Powerful A Subaru Impreza Can Go Up To?, (BHP) How much is humanely/mechanically possible?
is............How deep are your pockets??
You can buy alot of power for the right price but as alot of the guys running big power on here use their cars for running around in then you have to have a good balance of reliability/ driveability.