From what I've heard from several well known Subaru mechanics with 20 + years in the trade (including RCM who won't just replace head gaskets anymore due to this problem)
Replacing head gaskets on a leggy well used engine when they fail or are on there way out, leads to bottom end failure within a week or two normally, there are countless threads about this on multiple owners forums, from SES to NASIOC,
I would not be happy with the use that Steve intends for his car to simply do a head gasket replacement in light of this.
Indeed you could at 330bhp just use a standard Sti sized restrictor pill and 2 port, better to save the money until you decide to fit a nice big Turbo and spend lots of money
