Its just that the number is relative. One thing I know about those housings is that it puts a cap on the turbo which more or less makes it work like a good XXXkw turbo should. So in your case, it does a good job of 250kw whereas a .82 3076 would do a very mediocre job of 250kw.
I think the only thing that is holding your setup back is your boost, which is why I made the comment I did. Auto is OK, your general setup is OK, S15 injectors are OK, no issues - nothing to blame (IMHO).. Run more boost and you will have it try to make more power, but I'll bet that it will start to choke pretty quick.
From memory we theorized a lot with ARTZ about his one, his is the 7 blade 76 with a similar rear to urs I believe. If I knew which thread it was in I would link you. It helped him move off the dirty thin and onto a good thing! lol