Righto laddies
Decided to finally compile some information of the boosted side of flat 6 life.
This going to be exclusive to the early (99-03 JDM/AUDM EZ30D, single port heads with cast alloy intake/variable intake geometry) EZ.
Intake:
In terms of intake manifolds, at the moment, the best manifold is what you have. The cast manifold flows well and because of the fact that Subaru were nice enough to port match the intake to the cylinder heads, it results in pretty nice airflow and great potential for forced induction. If anyone has some conclusive data on upgraded throttles, politely let me know and we can add it here.
Fuel Injectors/Regs:
Injectors, are a little... different. For emissions purposes, Subaru added air assist to their injectors, to heat the cats faster. So, this means that standard top feeds won't work without deleting the air assist. Thankfully, Radium have us covered, naturally, with a part designed for Toyota's. The Radium Toyota air assist injector seat kit, allows the usage of standard top feeds, like the Denso STi Pink/Dark Blue injector, or larger, depending on your goals.
Regulators are not very different to any other Subaru. You can either run an adapter off the passenger side fuel rail or make a set of fuel rails with some fuel rail extrusion and a friend that has a milling machine and imperial thread cutters for that AN6/8 goodness 
Fuel Pumps:
This is, for the most part, self-explanatory and actually dependant on your build (whatever car it may be that you're shoe-horning a H6 into, then strapping a turbo to). Walbro or whichever, lots of LPH. The usuals.
Exhausts:
Ha, now, this is the fun part! For any Subaru EZ motor, there is no currently available, bolt-on turbo exhaust solution. This means, off you trot to your local fab shop OR, if you can weld, time to head to your local exhaust pipe supplier and stock up! As for sizing, we've often found that a 2 inch run off the heads to a collector, to an either 2 inch or 2.25 inch (2.5 is also acceptable, but smaller retains velocity) up-pipe works well. Turbo-back, is up to you. 3 inch or larger, however, is strongly recommended, given the motors displacement and typical turbo sizing.
Turbos:
Alright, the fun part. Now, I've seen a lot of people discussing turbos on H6's and unfortunately, many people exists under the interpretation that smaller, equals faster spool, which is absolutely correct! However, there is ALWAYS a trade off. H6's flow a considerable amount of air, that only increases with RPM. If you have a turbo that spools at 2000RPM, it's extremely unlikely to be in it's "efficiency island" by the time the H6 is at, and beyond, the peak of it's natural torque curve. What does that mean? Things are not good. For one, you're over-working the turbo. A lot. This turns the air flow into sheer friction, causing IAT's to increase dramatically, and not only that, the motor is now being choked by a turbo that ran out of flow 1000RPM ago, creating pre-turbine backpressure. Nice and, well, disastrous for your motor experiencing extreme asphyxiation.
Do your EZ a favour, run a decent sized turbo. For a single turbo application, we'll use Garrett sizing for the basis, a GT3071 is the absolute SMALLEST you would want to run on this motor. That would provide the strongest response for low-RPM driving, aside from a set-up utilising twin turbos.
Engine Management:
Now, much like the fuel pump arrangement, this is dependant on the platform you're using. For the most part, we're all building older cars without CAN, so practically any modern ECU can run our motors. The single best advice I can give here is, find a tuner, who knows the H6, and ask THEM about what ECU they like to tune, and recommend. I'm personally using a Link because my tuner likes and knows the Link platform well.
The next bit of advice here, use this time of upgrading to replace old and worn sensors. Coolant temp, things like that, benefit infinitely from being replaced.
If you're installing an aftermarket ECU however, it is not going to be a very easy plug in affair, unfortunately, as the 30D has minimal support still. Very doable however.
Engine Internals:
Finally got some worthwhile information to add here.
First off: Rods. These are the exact same as the later (04-09) EZ30. Which means, there is a fair few options. I have PPM I-beams for my engine, but the now popular Pauter rods are still an option, albeit an expensive one. The PPM ones run about $1800~ at last look and are rated for much more than any factory Subaru drivetrain could handle. From what I was told at my last visit, an estimated 500HP per cylinder is on the cards with those conrods and they're lighter than the factory cast units. He also includes genuine ARP fasteners.
Pistons. Seems the options seem to come and go with MCM viewership.. lol. I got my JE Piston forged 9.0:1's from Possum Bourne Motorsport, however, they seem to be NLA. Weird. Supertech are still available and if you wanted some custom JE's or similar, your local specialist speed shop could size a set up, no doubt.
Headstuds. This is an annoying one, as the factory EZ30R upgrade ones won't fit due to them being 10.50mm and the 1st gen EZ30 is 10mm. However, the custom ARP ones that are 12mm will work fine. SEND IT!
Main and Rod Bearings. Both available from King Racing, ACL and Subaru themselves. This is something of which you should probably know sizes first.
Valves/Springs/Retainers. Supertech has a number of options available for everything.
Buckets. Shimless buckets now available from Supertech, this is a desired part for those who might chase higher RPM.
Now, unfortunately, I don't have a lot to discuss in terms of engine internals, but if you guys would like my opinions on that, let me know.
Feel free to add suggestions/clarifications, happy to help. We're a small community, after all.
