Drilled & slotted rotors add zero performance on the street, but they do look nice.
I just installed front and rear disc brake kits on my VW dune buggy. The kits are made in China and have a spotty record for quality. Unfortunately, the made in the USA and Germany kits were three times the price, so I took a gamble and went with the cheap, Chinese made kits.
First off, the bolts that hold the caliper to the caliper bolts were the wrong thread pitch. I had to go down to my local hardware store and buy some bolts with the proper pitch.
Second, one of the rotors on the front kit was REALLY warped.
I made a short video of me spinning the rotor:
https://www.youtube.com/watch?v=zCaXEoq-3osThe place that sold me the kits is replacing the rotor, but I'm out the cost of having a machine shop press in the races and drill/press in the ARP studs.
The Chinese kit was $400, while a front disc brake kit that's made in Germany is $1700.
If this vehicle was my daily driver I would have spent the extra money for the quality kit, but I didn't want to spent that much money on a vehicle that I'll probably only drive a couple of dozen times over the summer months.