Hey guys I would like to try something wacky like put an EJ25 engine in my impreza which currently has an EJ15... Is it possible?

Yes.. It will drop right in no problem.  Engine management will be a problem though.

Problem how?

you will need the ecu and harness for the conversion.. Or find a piggy back/stand alone.  The map for your car will adequately fuel the bigger engine.

What you need to do sir spike is a sh*t load of research and reading on your own, it is all on the internet. This generally isn't the type of site to ask such a general question like that. Of course it will work but there are modifactions and other things you need to because the car did not come stock that way. Another thing that would be more helpful is to post your question in the appropriate section i e 90's New generation not in members rides. I understand if this is your first time here, just want to set you on the right path.