To notice any real difference in braking with an economy car like a Corolla, you'd have to be driving the car like a maniac and constantly be pushing it to the limit. For that little benefit, in my opinion it's wise just to leave it the way the engineers designed it and avoid all the headaches, expense and hassle. Drum brakes aren't so hard anyway and generally are much easier to do than a rear disc brake system and having the hassle of dealing with the parking brake mechanism.