The speed control can go in to sensorless mode if settings are too aggressive. This will cause the car to run much slower, as the timing functions will not be working (in sensorless mode)
For example, a 17.5 with timing might be geared around 5.5 - 6.0. A non timed 17.5 would be geared under 4.0... so in sensorless mode, without timing, it will obviously be very slow.
I have had this happen with a 10.5 (duo 2) once or twice... too much motor timing and speedo timing. I lowered the motor timing (2-3 geaduations from the zero mark, was previously at around 5) and lowered the gear ratio (7.5 to 7.0). This solved the problem. I let the motor do more work instead of the timing...
Also, there is no advantage in using the 120a in stock racing over the 60a, otherwise there would be no Xtreme stock esc....