I’ll address your questions one at a time:
Q. What accuracy can be expected from an ordinary user?
A. The technical specs for the Tobii X2-30, which is similar but not identical to the Tobii REX, state an accuracy (“offset error”) of 0.4 degrees and a precision (“noise”) of 0.32 degrees in an ideal environment.
If we take those values as a starting point and round up — to cater for hardware differences and perhaps a less-than-ideal environment — then the recommendation would be to design for an error of roughly 1 degree, corresponding to 1 cm at 60 cm distance. Which is in line with my experience.
Q. Can I use the TCP/IP firmware with my REX?
A. Since I happen to know that your device is from the very latest batch (it came with the 1.1.4 firmware on it), I wouldn’t recommend a downgrade. But the plan is to make the next firmware version support both USB-HID and TCP/IP, so please just have patience. And no, I don’t know when the next firmware version will be available :-).