INASNT & rev210,
don't forget that both Garrett and HKS (basically garrett) rate the supported power levels of their turbo's at 1bar of boost.
Also, as mentioned, if they 'apparently under-rate' the power levels of their turbo's, ~250rwkw might actually be possible at 1.4 - 1.5bar (~20 - 22 psi) along with modified cams, decent intercooler and very very fine tuning which i believe is most important and of course the fuel system to support this boost.power.
But again, as mentioned, although possible this would be running the turbo way out of its efficiency range, so u could go for something like this and push it hard to get your power, or you can get something slightly larger and make the power easier and more efficiently.