Cross drilling was first done by a BMW race team that was trying to make a very specific type of brake pad work for them. The problem was gases that were being released from the pad under braking that were causing problems. To release the gas, the cross drilled. It looked cool (no pun intended) so people started copying them.
Since no street pads have this outgassing problem, the cross drilling decreases the brake capacity.
Think about it. A brake rotor or drum is basically a heat sink. The kinetic energy of motion is converted to heat upon braking. The heat is transferred to the rotor/drum and then to the surrounding air. This makes the rotor a container and conductor for the heat. Its mass and thermal properties determine how much heat it can contain and transfer. When you decrease the mass(by drilling holes) you are decreasing the amount of heat that can be stored by the rotor.
I'm sure that the BMW team took this into account when drilling and used a larger rotor with more mass so they had some to give up.
Summation. For your street car with streetable pads, cross drilling is strictly cosmetic.
Have a great day,