Fiestas are already being made in Thailand, so it's not a move as it is closing their Mexico operations. With the next model (8th I believe), they are going to increase capacity.
The ST lists for $22,200 while the Sonic RS lists for $21,000. To me, that extra $1200 would be worth the extra 50HP (197 vs 148) and according to all reports, a very nice, dynamic, and superior to the Sonic, suspension setup. Downsides, the Recaro seats are an extra $2k and no one is, or should be, paying list for a Sonic. And the 1.6L Ecoboost is not very full efficient. Although not rated yet, I'd be surprised if it rates at 33mpg highway. Lastly, the ST is hatch only, and the LTZ 1.4T sedan is still damn nice. But for the difference in costs, you're not going to make a Sonic beat an ST in acceleration or handling.