there are allot of different aspects when it comes to a rotor.
What is it made from (Iron, Steel, Aluminum, Carbon ceramic, Carbon Carbon)
Where is it made
Is it 2 piece (hat made from aluminum, Rotor ring made from iron)
if it is 2 piece does it float?
Not wanting to spend all day i'll try to keep the answer short. and focus on the standard parts store rotor.
Standard rotors that you find on Rockauto, Autozone, and so on come from china. They are covered and machined with oil, the material to make them is not as pure as it should be, and are never made engineered for racing. allow me to explain.
The machining process for Chinese rotors involves blasting the machining surface and tools with a oil to keep them cool, this lowers maintenance costs on machines. Then factory workers will warp the rotors with an oil impregnated paper. This oil is in an attempt to prevent rust, at that it works well. The problem with using so much oil is that, it will impregnate itself into the iron. once that iron is impregnated the oils will come out under extreme braking. Most of the time when you see issues creating and keeping a transfer layer, the oil inside the iron is causing that issue. It impurities in the rotor are kicking the transfer layer off. Racing companies like PFC, Brembo, AP, will cut the rotors "dry". Dry means to machine the parts without any oil. it is more expensive to do this but will increase the rotors ability to obtain and keep a transfer layer.
Next is what it's made from. Iron is easily one of the best materials used for rotors. Size for size an iron rotor will outperform a carbon ceramic rotor. if you have 14" rotors one in carbon ceramic and one in iron, the iron one will generate a more consistent and higher TQ output. Not to go too off track but that is why you see 15-16" carbon ceramic rotors on oem vehicles. The oem manufacturers need the increased size to get the performance. but Carbon ceramics will outlast a iron rotor. ...sorry back on topic, right iron.. Iron is one of the densest materials in the universe. also how pure it is in the casting for a rotor makes a considerable margin in braking performance. rotors that are casted in china are known to have quite a few impurities, and are labeled incorrectly. anyone that has dealt with 304 *chinese* stainless will know what i'm talking about. these impurities will have a decreased effect on braking performance. I like to find Rotors manufactured and casted in the usa. the us has a higher standard of casting. again PFC, AP, and brembo all are manufactured in the USA, GB, or Italy.
finally the engineering in the rotors could take year to explain, so i'll focus on balancing only. when a rotor gets hot it cones, it beds, and it looks like a wave. it's never still. The material is constantly flexing. think about a top fuel dragster tire in slow motion.. it's not 100% the same but it is close. that rotor is constantly trying to keep itself together and not explode. in order to stop a rotor from shaking the wheel all manufacturers will balance them. Top Motorsports brake manufacturers will balance the rotor by cutting the entire outer perimeter of the rotor on a lathe. this ensures that when a rotor is at thermal capacity, there will be no places of excess or minimal material. it makes the rotor stronger, and more resistant to cracking. part store rotors balance the rotors by finding the heavy spot on the rotor and cutting that off.
what not to do.
In Motorsports this is a huge no-no. the hard edges give a place for cracks to start, the material has a thin spot on the rotor, and it means that the material that is spinning is not balanced through the assembly. it can lead to a cracked rotor and can lead to problems with a long pedal.
In conclusion, :P. is high quality rotors a waist? Like tools good quality parts are never a waste. On average a motorsports rotor will outlast a parts store rotor long enough to justify the price. Parts store rotors can work, they also let allot of people down. If it was my money, paying for my track day, i'm putting the parts on my car that will insure that i have a fun weekend. i'm not looking not fight the $30 rotor that costed me $800 of track time. Allot of people might say "they worked fine for me". Truth of the matter is 80% of the motorsports population knows how to build a motor, 10% know how to build a braking system. when the brakes that they said "worked fine" didn't work "fine", and that person has no idea what the problem actually is.