um yes it is suppose to leak
all of the usdm legacy turbos do and my G version does too
any way that shouldn't affect it
the twin turbo is generally out of your realm as it won't clear the LHD parts of the car
that said, ill throw out
try fuel injector cleaner?
how good is your ground strap?
most all subarus produce peak power at 5250 or so so this is when the car is working the hardest