SD card speed class specifies "minimum sequential write speed" but not the "maximum sequential write speed", why?
To be considered to meet the requirements of the relevant class the card must support writing at a given minimum speed (eg. 6MB/s for class 6). If they are faster, that is okay. It's like saying you must have a minimum income of $150K to qualify for this credit card, if you have more that's okay. 

Class 0 is not rated under this system so you would have to check the manufacturer's specifications if you want to use SPI. 

And you should not necessarily expect that the average write rate of the card means that you can write every byte at that rate. You may have to have a sizable buffer to account for the card going off on its own from time to time.