Unfortunately, it looks like I'll need to implement syncstorage myself, with my own back-end. In PR2, an application which works well with localstorage randomly breaks with syncstorage.

Data is lost, the amount of records returned and available after the 'load' event randomly varies, and random errors are thrown within sencha-io.js. Given a store with 4 entries, I would receive 4 entries on the second load, 1 on the third, 5 on the second, and then at some point, all records would double during a load, and then return normally on the next.

The experience was extremely inconsistent, and my attempt at simple cloud data sync was a complete failure.