you can, but its hard.
for 250whp, you need a 2.5" exhaust all the way back, FMIC, unichip, exhaust mani, 440cc injectors (i think), intake mani would help, EBC, boost gauge, wideband, and a wicked tune. a larger turbo would help considerably, as the GT25 is out of its efficiency range at redline above OEM boost. thats not to say its a bad turbo, its adequate, but youll make more whp on a bigger turbo at the same boost - plain and simple.
all of the above would probably make 250whp @10psi on a GT28rs, assuming you have a good tune, and the knowledge that youre pushing engine limits at 250whp.