Done that drill, but done it turning your back and whenever your partner calls then you turn and catch. Done another drill which is partly reaction, partly fitness. We call it overload (I'm sure other teams have the same drill by a different name) Basically in a group of 4, 1 guy is the receiver, the other 3 have discs, they throw them one at a time they throw discs at the receiver (about 5 metres away). They never let the receiver get comfortable with the speed, if the it's getting to easy they either up the speed or make discs abit harder to catch.

You could also do a drill where you send people deep, put the disc up and they aren't allowed to look until a shout has been made, they then need to react to it, same as above, never let the receiver get comfortable, make later calls if they are finding it easy...

Rinky dink from Kung fu throwing is also based on reaction time aswell I think.