because we have Macpherson front struts there is a loss of camber in cornering, therefore you will have horrible cornering grip, not to mention major body-roll.
That is one HUGE difference I noticed in my old WRX stock compared to a larger front sway bar, and with my MP3 having a 25mm front sway bar it is nice that I don't have to upgrade for Auto-x.
You may not notice much difference just putting around the block, but anything aggressive and you'll notice. The oversteer will occur since the rear is now unbalanced with the front, ie: its stiffer so it will come around much more suddenly.