After running multiples tests at a reasonnable speed (Feedrate : 15000 mm/min), I can tell that the motion is continuous between multiple G0/G1 commands!
By the way, I don't understand how the "T" parameter from the M670 works (when using the "P" parameter in a G1 command). I thought at first that is was the delay between the beginning of the movement and the beginning of the output being set to '1'. But when testing with multiple values for "T", the output was always set to '1' immediatly after the beginning of the G1 command.
Edit : The "T" parameter appears to be important when using the P1 parameter with a G1 command on a very short distance. Reduce the "T" value if you're looking to trigger an output to '1' on a short distance and high speed (30 mm and 15000 mm/min).