Putting a WRX hood or a STI spoiler on a 2.5i is without a question rice. That being said, i just bought a WRX hood and will be putting it on my 2.5i as soon as its painted.
Yes, its rice and served no purpose whatsoever other than making my engine dirty, costing money and probably increasing drag, but i personally couldn't stand looking out at that bland nothingness anymore and went for it.
As for the spoiler. I have seen the 2011 STI wing on a WRX and it looked like it fit rather well. Not exactly my cup of tea but it did work. the deck lid spoiler is a good compromise if you dont want to show off too much and like that subtle look. I however prefer the clean look and went with no spoiler and dont intend to get one. Wingless all the way for me
but at end of the day, its up to you