here is the definition of SD class rating from sdcard.org. Nowhere i can see that class rating refers to random read/write so i would say since the cards meet 4MB/s in your ATTO benchmark then no RMA would not be possible. also depending on the technology used in the card performance can vary.
http://www.sdcard.org/developers/tech/speed_class/
SD Speed Class/UHS Speed Class
Greater Performance Choices
There are two kinds of Speed Class, “Speed Class” and “UHS Speed Class.”
As a characteristic of flash memory, actual transfer speed varies. Variable speeds are difficult to reliably record streaming content such as video because it requires a constant writing speed. Speed Class and UHS Speed Class provide the constant speed necessary for video recording by designating a minimum writing performance so that minimum and constant speed is guaranteed for camcorders, video recorders and other devices with video recording capabilities under the conditional write operation specified in the specification.
Speed Class, designated as Class 2, 4, 6 and 10, is designed for normal and high speed bus interface (mode) and UHS Speed Class 1 is designed for UHS-I bus interface*. (Speed Class and the UHS Speed Class are not compatible.)
