Hi Joe, just to let you know my car just made 480hp today on a G25-550, so on that basis your 660 should make 580hp
I went with a Pulsar turbo which is a really high quality exact copy of the Garrett original. It is half the cost and has a lot more A/R and housing options than Garrettt offer. So if you havent bought your turbo yet I would recommend that you have a look at Pulsar's website. Really good customer service as well