So after some research, I don't believe the factory Y-pipe is restrictive at all, the combined diameter equates to just over 3" at the rear which is standard aftermarket turbo exhaust outlet for most turbo cars including those making big power. The size of the mid-pipe (haven't measured but looks about 2.5") and the factory resonator would probably be the biggest restriction but I can't see this being a restriction until over 200atw and that is just based off other Subaru's I have known and the size of the exhaust (some of the old lib boys used to run 2.5 at the rear and found it to keep more bottom end without really sacrificing top end (tuned exhaust). My old 3" Fujitsubo system actually is 3" but the outlet to tip is 2.5" increasing to 4".
So I have been thinking about this a lot lately as I want to replace at least the dump pipe but didn't want to have to custom modify anything (bolt-on only). What I have realised (and stand to be corrected) is that there is simply no sensible bolt on replacement except for doing the whole exhaust including mufflers. The reason being that somewhere you will encounter a step-down (blunt edge) as the aftermarket component meets the smaller diameter stock exhaust components. This will actually be much worse at the front of the exhaust where exhaust gas velocity is higher (e.g. if you just replace the dump). However after comparing the size of aftermarket Y-pipes they exit at a minimum of 2.25" whereas the stock muffler inlet is ~1.5" which is actually a horrible step and disruptor to have in your exhaust. I have spoken with Chris from Best Mufflers to explore options and he agrees he simply wouldn't mate an exhaust that had a blunt step. If you were replacing just the mufflers however this wouldn't be an issue as you are going from smaller to larger in direction of airflow.
So given I really want to keep the STI mufflers and they won't be a restriction for my power goals I simply need to have some customisation of my exhaust
. I have also looked at the pictures posted above of the stock exhaust and it actually looks very clean, the size I don't believe is a restriction for my power goals and Chris reckons he has cut open one of the factory resonators and they are a straight through design so not as horrible as I might have thought.
Given it is too tricky to try and modify the y-pipe with a downsizer and likely alignment issues I have decided to replace dump-pipe only. Problem is I want one of the TCPs or iNvidia and both have CAT in downpipe which means there won't be any gap between where the down-pipe meets the factory exhaust so the only way to create a smooth transition would be to hack the new dump pipe (I don't think so) or modify the inlet to factory mid-pipe. I have a picture below that depicts what I am attempting. I expect the mod to cost a couple of hundred if I get flanges fitted so I can revert to factory without a return visit to the exhaust shop. That would pay for the flanges and labour. I would of course need to fit the dump pipe myself and do a temporary bolt up to get to the exhaust shop. Thoughts and comments welcome.