[sybir]

No, you got it - only the EJ207's had AVCS in the EJ20 family. The US STi heads aren't worked quite as heavily as the JDM heads from what I remember, but they're not *really* a restriction until you've got way too much money in the car. AVCS helps fatten the torque curve, but not nearly as much as the displacement does - a lot of the higher-end cars actually ditch AVCS for simplicity's sake.

I'm really not sold on big head upgrades for most street cars. Sure, within reason, there's limits, but if you have a turbo that can make enough boost at any reasonable RPM to require a wastegate to limit boost (in other words, it's not running out of oomph within its efficiency range), you're pushing a lot of air through whatever orifices are available, and smoothing, opening, enlarging, and streamlining is a final step on a fully prepped car to get the last 2-3% - at least on most parts ahead of the wastegate/exhaust outlet (non-pressurized components are a completely different world and are why a good NA build takes so much comprehensive work to so many systems. Flow characteristics, turbulence, etc - they all matter and can be improved, but you're chasing diminishing returns, especially on a street car. Most headwork, and upstream exhaust work, by association, helps spool, and therefore area under the curve, more than it affects ultimate power. Now, AUTC is what most of us care about, especially on street cars, but a lot of time it's going to come down to "feel" mods and perceived changes rather than calculable numbers.

I mean, I used to do the shop thing, I'm not saying there aren't a lot of bits that will get you another 2-3% (and on a 200-300hp car, you're talking 5-10hp with some of the more problematic points addressed, but it's diminishing returns on a stock-type turbo that's flowing reasonable amounts of air and pressure. Picture a garden hose. Put your thumb over the end and fill it with water slowly; you can hold it until it fills up. Turn off the water, and you can really control the flow rate out of the hose. Now, try jamming your thumb over the end with the water on - that's a simplistic way of looking at intake (primarily) and exhaust port flow on the turbo car that's making non-trivial boost. The air is going to get shoved where it needs to be because physics dictate it has to go somewhere. Once the system is pressurized, flow characteristics become moot under most normal conditions.

If you're reading that I'd rather build a stouter bottom end and a little more boost than try to finesse every .25 hp out of a motor, you get the picture. Within reason, a few pounds of boost will make up for sub-optimal flow.