i built my 2.5 back in 2004 and feel a 2L wil never do again,certainly not for reliability on our miserable irish fuel. you do need a little more outlay with the 2.5 for a safe running engine but its well worth it. with a mild map, 300 to 320, you will enjoy the increased driveability and wont be paying any more at the pumps thanks to the extra torque.
i plan building another 2.5 very soon. the latest 2.5Sti is rated as 20% stronger.