The critical speed for these 16mm screws (assuming a 13mm min diameter) is over 2700rpm, thats a whopping 27m/min. I think 16mm will be OK.

Thanks for the stepper driver advice JAZZCNC.

How do you guys attach linear rails to a steel frame. Do you just drill and tap straight into the 3mm wall?
An M5 screw has 0.8mm pitch, so will be over 3 full threads in the steel, which is the rule of thumb iv come across a few times.