The swaybar links are identical between the OB and other Legacys of the same body styles. Same with sway bars (other than size).
We have an 18mm bar on my wife's GT. She's not a performance driver by any means, but has not had any problems with the car oversteering. The ride's a bit harsher, and the car tramlines a bit more, but that's to be expected - the stiffer the swaybar, the less independent your suspension is. I think you'd be perfectly safe putting an 18mm on there.