Well, if you move mouse too fast you get max 127 counts in each direction which results in inaccurate measurement due to some not being recorded.
According to wiki ps2 interface, the range should be -255, 255 - it's 9 bit two complement intiger.
However ps2 library uses chars as variables which are 8 bits. Any solution how to add that 9th bit? This should result in doubling maximum distance before going out of range