FYI- The truth behind cross drilled rotors is often misunderstood. First and foremost, it has nothing to do with cooling; that is a misnomer. The original intent of cross drilling rotors was solely for weight saving purposes. Porsche was the first to try this back in the early 1970's at Le Mans with the 917's. At that time Porsche was drilling holes in just about any piece of metal they could without sacrificing structural rigidity. Believe it or not, they even drilled holes in the key. It may not seem like a lot, but added up, every ounce counted, which can make a difference of a win or a loss over a 24 hour race.
In the case of cross drilled rotors, what Porsche found is it also added in the dissipation of gasses that built up from the brake pads which was *one* of the causes of brake fade in the early stages of the race. When new pads or "non-bedded" pads are used hard for the first few times, heat causes the adhesives, used to bind the brake pad material together, to release gasses. These gasses build up between the pad and rotor causing the pad to "float" and loose much of it's braking force. What Porsche found out that in addition to the weight savings from drilling rotors, the holes provided a means to evacuate the gasses and minimize brake fade. However, over time, they found out the down side of cross drilling rotors was it severely compromised the structural stability of the rotor. Throughout a race, brakes would "heat cycle" several times a lap going from glowing red-hot
to much cooler temps. This caused the rotors to expand and contract over and over until the metal began to fracture. In some cases, the rotors would literally "explode". Fast forward and modern brake manufactures now know they can achieve the same "venting of gasses" by slotting the rotors, without sacrificing safety. Please reference the links in my previous post for more details; I'm too lazy to do it again.
If you drill holes in your factory rotors, you do so at your own risk.
If you look closely at a "vented" rotor, [which the Sonata has] they are actually *two* discs, sandwiched together with cooling vanes cast in between them. See fig [A] below. These vanes act like fan blades, pumping air between the discs to aid in cooling. The problem with drilling a factory rotor is it's almost impossible to do so, without hitting those vanes which further decreases the strength of the rotors. In the case of the cross drilled rotors found in most "big brake" upgrade kits, they are actually rotors from Porsche. Porsche took the time to "cast" the holes in between the vanes to insure no loss in structural stability. Even so, they too still develop small but visible fractures in each hole. See fig (I have personally witnessed this when I ran them on my Porsche race car.) Some race clubs have actually begun to ban crossed drilled rotors for this very reason; while even more are considering it.
In conclusion, I'll be the first to admit, cross drilled rotors look very cool and if all you do is normal driving on the streets, you'll probably be OK; but that's a lot of money to spend on just looking cool. That said, if you intend on doing any *actual* hard driving, your much better off with slotted-only rotors.