I have a 27mm Whiteline on front my WRX and 26 in the rear. Mine are adjustable though: My front = 27,29 and rear = 22,24,26
The best way to see if something fits is to find a place that sells it and check out their application guide. http://www.gruppe-s.com/Subaru/subsus.htm. On the gruppe-s it says that that bar is made for any 02+ WRX. So yes, it does fit.
They do have a new 27 rear bar. I only suggest that if you do a lot of auto-cross. Savsuby, the 27 front bar is always good (as long as you have a sedan, they don't fit the wagon) The 22-26 adjustable rear is normally the match. If you do alot of auto-cross it might be worth trying the 27 rear. SS