Increasing the ability of the front brakes is the most effective way to slow the car. While the car may have more stopping power the attitude of the car (suspension) will more dictate the oversteer of it. Being able to stop deep may put the weight up front sooner and more nose dive may be in order, but given that the rears won't lock up and skid and that most FWD cars reak of understeer, this can be in fact a bit of a help!
I've done a rear kit for the car too and it's harder as you certainly don't want to have them lock up. Want the car to stop better AND use the rear brakes more? Put two of you best buddies in the back seat and sand bags in the trunk.
Oh, and the shims are the spacers to fit the alternate calipers to the same bracket. Each of them has different size 'ears' and thus need to be centered on the rotor.
