the only exhaust leak that would matter would be between the heads and the bottom of the up pipe
and the 04 sti has no air pump
and we should be looking at any parts that have not been changed since all these engines have been blowing up
look at the harness for the front o2 sensor for broken/burned wires
the ecu maybe bad
stop driving the car unil you get it sorted as it will blow another engine here in a minit if you do keep driving it