you mentioned about buffering capability limitations. I feel that too. What happens in case of larger sketches? or not so reliable internet connections? Isn't it safer to save it to a local memory before?
I see the spark core has a on board flash memory, "SST25VF016B-75-4I-S2AF-T" of 16 mbit. Can I assume that they used it as a buffer? or as I was initially thinking, the sketch is downloaded and saved to the local memory first before flashing.