Problem is quite a few people stick a bigger turbo and still only get high 12's. As they have neglected much of the handling and support issues like good tyres, handling items, etc because they're dont want to waste money on that.. but when you consider the total costs (AFM, ECU, turbo, fitting) there is not much way to get it all done for much change out of $5k. That buys a lot of other stuff. But yeah, big turbo gives the most instant hp no doubt about it.. but if its just "bang" spin, spin on the wheels.. stupid.
Other option to the HKS 2530 which is looking nice is the Trust 517z (for around 230rwkw).. or 518z (for around 250rwkw).. Pretty much same torque curve, just the 517z drops off a little earlier. Depends whether you believe in trusting plain bearing vs ballbearing. Trust has been building plain bearing turbos for a long time, obviously they see no need to go for ballbearing. I've been looking a little, these turbos can be had for under $2500 shipped from japan new. Not too exxy.
lol - you've got a silvia anyway! what u talking about.. different turbo combos you need for the silvia. Generally you can get away with smaller for same power as the rb.