From what I have read, if you do the front you need to further thicken up the back to compensate for understeer.
You probably read that from people who don't know what they are talking about.
"ideal" is using sway-bars as the final piece in a suspension puzzle to tweak everything. Here, the 5th gen is at a disadvantage. I don't believe there are any replacement strut options out there, which makes suspension optimization even more difficult.
Swaybars reduce body roll by increasing the effective spring rate as weight shifts. With stock struts, this often makes a typically oversprung suspension much worse. Not an issue on glass-smooth on-ramps, but murder on bumpy turns you want to take at speed.
Simply tossing a big RSB on a car does not reduce understeer, it simply increases oversteer. There is a subtle, but critical, difference between the two.
If you MUST do something, the STi RSB is probably the biggest I'd go with otherwise stock suspension. I'd absolutely invest your time and money into endlinks and a proper alignment.