The stock LS rear is 14mm I believe. Putting a larger rear swaybar in the rear will make the car more prone to oversteer (as you have already found out... "kicking the rear out"). Most of us like oversteer--or at least more oversteer than Honda gave us (none). This is why I use the smallest swaybar available in the front (5g Si) and the largest available in the rear (itr). The handling difference takes some getting used to, but is very rewarding when driven well. You will have to be a little more careful with mid-corner throttle adjustment and braking so as not to spin the car.

But as Mike mentioned, allignment can have a huge effect on the handling balance of your car, as can the presence of worn bushings... so get both of those checked if you think its a problem.