I agree completely. And I have debated that g30. But I haven’t found many results on it. I know the gtx line is proven to take abuse and hold up. So I’m leaning more towards a gtx.KB has done a lot for the community and has always been a good source of information. The GTX3576 is an awesome turbo for that "500-550ish whp" range you want. I believe taking a look at Garrett's new G series as well may also be a great option for you. The G30-770 looks like it will give the GTX3576 a run for it's money with both spool and Garrett themselves rate it for more power potential. Then there's the G35-900 which is the same size as the GTX3576.
and yes killer b has done quite a bit for our subaru community for sure. I wish they still made their low mount kit lol