I notice that all the chinese rotors seem to have less material in the breaking surface when compared to Shimano/Sram. Wouldn't that affect performance/heat dissipation and perhaps mean they wear faster?
Heat dissipation is mainly affected by the heat sink area - the more the area, the better is heat dissipation. So highly perforated rotors have great heat dissipation. I've used KCNC Razors for years without any overheating problems (on descends with less than 500m of vertical drop), and now using Chinese rotors of the same weight (107g @ 180mm, 76g @ 160mm) with the same result.
But breaking performance
is affected by amount of material on the braking surface, so there is a compromise, as anywhere else in the bicycle.
And of course, the less the material on braking surface, the faster the brake pads and rotor itself will wear out.