unfortunalty the 2530 will puff short of 250rwkws. however as steve already mentioned, due to the rb20 having a lower CR, the engine wont mind some more boost over 1bar, and hks turbos are really efficeint still after 1 bar, so it may get really close thou.
The 2835 will get u over that mark, but it will be a lot more laggyier then the 2530, and being a rb20, u will feel it alot.
Perhaps speak to a few about a 2540, a bit laggier then the 2530, but should also be a trade off over the 2835. Otherwise choose a 2530, and do other mods that may help u get there, like adding toluene sometimes when u want to use all the power.