I don't read the turbo tech forums, because I don't have a turbo and may never bother to get one, so if it's been covered in there, sorry. I haven't seen it discussed anywhere. I also don't do a heck of a lot of reading over on the i-club anymore, just the nwic and off-topic, with occasional jumps over to the others. Basically it's not the i-club anymore, it's the wrx-club, and there's not much info worth reading.
On my previous car, I went up 6mm in TB size. I noticed a difference in throttle response and a noticed gain in upper RPM power. If someone wants to see if the H6 TB will work, cross reference the gaskets. If the bolt holes match, you're good. I would bet that the sensors from the stock TB would cross right over too.
OK so I was reading Cobb's tech articles, and the stock TB was within 1% of the stock intake (plenum, heads, whatever) and so messing with the TB wouldn't really benefit as much as it would cost. Do work on the rest of the intake, then worry about it.
Trey Cobb says on his website that the main weakness of the EJ25 is the internals: heads, cams & intake manifold. You probably won't see any significant N/A gains until you modify those. Also, when cops, smog officials, or street-racers (can we say that here? Oh, yes we can, it's not the i-club ) look under your hood, they won't notice anything out of the ordinary. Only problem is that it costs so much. After the price of labor, you'd have spent close to $4K for an improvement of less than 75HP to the wheels. But after all that's been done, all your other breathing mods (intake, headers, exhaust, throttle body) will be more effective. With all the bolt-ons, plus cobb heads, cams, & throttle body, you should be faster than a stock WRX.