The rear end shouldn't want to step out at all with the stock rear sway bar on there. Dropping the throttle mid-corner should get the car to tuck in a little bit, but not actually transition to oversteer. If you're actually getting the rear to come around, I'd check your alignment - make sure your rear toe is at zero. A little toe out in the rear will make the rear more willing to come around - I used to run a bit of toe out for auto-x.
With both of my GTs, the stock suspension + 18mm bar was very nice. On my '97, stock suspension + 18mm bar + Kartboy endlink bushings + Powerflex rear sway bar bushings = absolutely perfect balance. The car would still push if I got on the gas too hard in low speed corners (ie trying to accelerate too hard at an auto-x), but at speed the car was wonderfully neutral.
So, make sure your alignment is good, and go for the 18mm. You should be fine.
'97 Legacy 2.5GT sedan