Hey Jeff, is that the same turbo that Benson is running on his car as well? How fast do you build 8.5 psi in that turbo?
And no, the s15 turbo is way different than the gt28rs. The inlet is different (but you might be able to get it made like a stock t25/t28 inlet). The S15 turbo, like people have said, is good to about 300whp with supporting mods.
The GT28RS could hit around 320+ whp if you have the supporting mods as well.
It would definitely help if you had cams though. 256/256 or 256/264 would be good for either one of those turbos.