I had trouble controlling Supes through these races until I realized that steering during super-speed flight should be done with the right-analog stick. The left analog is overly sensitive during super-speed and only controls turning anyway. Use the right analog unless you have to drop out of super-speed for a second (if you accidentally go off course), then use the left-analog to get out from behind the building or whatever until you resume flying at super-speed..
Once I started steering that way, these races were pretty easy.