I know this thread doesn't get much traffic, but thus far I'm pleasantly surprised by the Fiesta's handling. Slow to accelerate, but it handles the canyon well.
I've purchased a rear disc brake kit already. I plan to add front and rear sways, some chassis bracing, and the Ford handling pack for AutoX this summer.
One member at the Fiesta Faction forum was 1 second behind a stock 2011 WRX in his stock Fiesta, so it looks like I might be able to ruffle some feathers on the smaller, tighter courses in Sioux Falls.