What I drool over is this:
Forced Performance Turbochargers: Subaru FP 20G Turbocharger
When I was shopping for an STi, I would have told you I'd eventually end up with a TD05-18G but after driving the STi for a while, you realize how ridiculous 300tq is at 3000rpms in a 3200lb car. Especially with the tight 6-speed, pushing the power band up a little higher is not that laggy. You come off of first at say 6500rpms and you're already at 4k in 2nd before the thing's even spooled. The heads on the STi really handle the larger turbos nicer than the WRX as well.
The FP Green is another excellent choice but for those who passionately hate lag (guilty), it's a little on the large size. It's a great turbo for those who have a more balanced approach though.
FP has a great discussion suggesting that for those who use pump gas, you might not want as big a turbo as you think as well. I tend to agree with this strongly. Basically, the "mosc" recommendations are (forgive the arrogance of that statement):
EJ205: TD05-16G (Evo3)
EJ255: TD06-18G
EJ257: TD06-20G
Built-up 2.5L+ EJ motor: GT30R or APS TSR70 (for those who are loaded)
Maybe up all of those a size if you intend to use race gas.
Oh, and no disrespect to the SZ-55 intended! You guys are building a beast. I'm sure farting around town on pump gas isn't the only thing you've got planned for your STi though...
) but they aren't so big that they will affect streetabilty and increase more spool time than what you want. With either of those turbo's you can run a FMIC though on the green/SZ-49 I personally would recommend going with a front mount, with the 20G it's your choice.
. It's a good point to put out that I didn't think of when you compare the 6 spd tranny's to the WRX 5 spd. They are quite different and if you haven't every driven an STi, it's harder to understand. 
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