2005-06 Legacy GT's have a catalytic converter inside the up pipe before the turbo, replacing gets you ~10whp and might save your turbo. I think Subaru realized this too and in 07 LGT's came with a catless same as STI up pipe, instead they introduced an air pump to help the emissions.
GrimmSpeed did some awesome CFM testing
of different up pipes. Catted up pipes flow 165 CFM, you can gut the stock one, very involved process and you run the risk left over material going into your turbo, airflow does improve by 50 CFM (216 total), which is still worse then OEM catless uppipe (at 245 CFM). GrimmSpeed's uppipe did 293 CFM, which is almost double the airflow of stock. Now Grimm's up pipe does have a flex pipe, which IMO are not needed and add turbulence, that plus the price is why I went with the Invidia uppipe.
Up pipe is install is not hard, but is also not easy as the downpipe install. It's just a very involved process where you have to remove the headers and the downpipe.
Gain access to the turbo and downpipe.
Remove the EGT sensor if you can, it will help you more easily pull out the up pipe. Mine was stuck thus I had to remove the uppipe as is, it was still doable, just bent the sensor.
Mine was so stuck I couldn't get it off even with MAP heat while it was off the car.
Downpipe has a bolt that holds it under the car, but there is also a little bracket that catches it. This was a pain to work with, I ended up cutting the lip off so that I can push down the downpipe more.
Remove the downpipe bolts. There is one downpipe bolt (one on the intercooler side) which was nearly impossible to break loose, just not enough room for a breaker bar and a wrench cuts your hands. Use a screw driver to force the downpipe off the turbo. This all is much easier if you just remove the downpipe all together and replace it with a new one. I don't know why Subaru thought it is a great idea to put two bolts right under the downpipe.
Next is removing the header, first you need to drop the diaper and start removing the super rusty heat shields, to gain access to the nuts on the heads.
Unplug and unhook the O2 sensor from the engine! O2 sensors are very sensitive on these cars, I damaged mine by leaving the exhaust manifold hang on the O2 sensor wires!
Don't forget to unbolt the up pipe from the header too.
This caught me by surprise, no wonder catless pipes gain so much power!
Grab it and pull it out
Invidia vs OEM catted up pipe header end:
OEM LGT uppipe entry is about 40mm
STI Upipipe is not much abetter at about 43mm
It should be well over 50mm to not be a flow wall, this is the exact reason why I like the Invidia Uppipe