i had the same problem,

they simply blame it on mods, my engine packed up previously and i had the exact same problem from day 1, im guessing ur dealing with melville subaru.

they tell ya its normal to brun that much oil and send you on your way. if possible, get your a standard exhaust and test it with the standard exhaust,

get a engineer involved and get some investigation done,

my warranty lapsed, my engine went and i was left with the bill to do the whole procedure.

Lazzo gotsis from subaru aus will come down and see you and tell you its ur exhaust and put it back on you.
get them to put it in writting and get compression tests done.

make sure you are there with them everytime they top up ur engine oil, so you can see how much goes in not how much they say.

when my engine blew and car was takin back in, they topped up the engine oil while the car was fucked, and i know it was bone dry, put simply fuck subaru melville - i would never get them to service my car specially melville!

put lightly u cant do much.

good luck tho