People are going with the 20g's on the Sti for the following reason:
1. With a bigger turbo you have to rotate it build the piping and for the power you are going to want to do front mount. AZ scoobie is doing gt35r kits for $3000. That basically the same price as BG but with a front mount added.
2. With the 20g you get fairly good power and you don't sacrifice longevity of the motor as much as you would with the bigger hp numbers of a bigger turbo. Sure AZ scoobie is a hell of a tuner and he has put 526 whp to the stock Sti block and its still holding. But I do not know for how long. It's definately more of a gamble than a 20 g is though.
3. With a 20g you can save money and keep the top mount. It's on the border for the effiency of the topmount probably though. So it is still cheaper.
4. Lag of the 20g is about the same lag of a stock evo. Not noticable at all and great for auto crossing and stuff. Which is what most of the suby guys do.

Dan