In parallel, the motor will run in a calm and dignified manner. In series, it will be nudging the safety margin built into its specification, and will need either a smaller prop or water cooling or both.
An ESC that is rated as "12 volt" by its maker is intended for a 12 volt SLA, and so is, or should be, good for the 14 and a bit that a fully charged one will deliver. I would look very closely at the specifications if fitting a battery arrangement that was going to deliver nearer 17 volts when charged. Maybe have a look at other threads in the forum.
I find it interesting the differences between series and parallel wiring in motors and batteries:
PARALLEL MOTORS
Pros - Max power, high top speed
Cons - Half the run time of a single motor setup or dual motors in series, high amp draw needing heavy duty ESC
PARALLEL BATTERIES
Pros - Long run time, easy on electrics
Cons - Heavy
SERIES MOTORS
Pros - Long run time, lots of torque
Cons - Low top speed
SERIES BATTERIES
Pros - Explosive power
Cons - Heavy, requires heavy duty ESC
What happens with parallel motors on series batteries? I am but a novice...