*I am unsure how much data the arduino can handle. I plan on sending a pair of integer numbers (4 bytes each) 60 times per second, so 240 bytes/sec. I believe (could be wrong) the arduino can only handle 9600 baud which is something like 150 bytes/sec. I believe a workaround would be to use 16 bit ints, which would drop that to 120 bytes/sec and still maintain an accuracy higher than the stepper would need.
This is the least of your worries - the Arduino can manage 115200 baud easily and people have run it faster sucessfully.