i learned something about GM brake rotors about 10 years ago when i was gathering parts to put C4 Vette front brakes on my 71 Nova.
a friend of mine worked at a GM dealer parts counter, so he could get me discounts on GM parts- like right at dealer cost.
they had a set of GM rotors for an early 90's Vette on the shelf- for $90 each with the discount. i priced a cheap set at a different parts store for $35 each- so i bought them. i took them to the dealer to compare them- they were in the same box with a different sticker on it. looking at the rotors, they were identical right down to casting numbers and stamped numbers. they were literally the exact same part from the same factory in China.
i used the cheap ones, and he said that they sold about 5 sets of the "genuine" GM rotors every week to Vette guys that had to have the good parts on their cars.
i can't say that i had any problems with the rotors- i put about 15,000 miles on that car over the next 2 years and the rotors still looked as good as new and didn't warp at all even with the "spirited" driving style that car caused me to have..