I figured out the solution finally. To update at 400hz the code must be changed to
#define MIN_PULSE_WIDTH 1000// the shortest pulse sent to a servo
#define MAX_PULSE_WIDTH 2000// the longest pulse sent to a servo
#define DEFAULT_PULSE_WIDTH 1500 // default pulse width when servo is attached
#define REFRESH_INTERVAL 2500// minumim time to refresh servos in microseconds
#define SERVOS_PER_TIMER 1 // the maximum number of servos controlled by one timer
#define MAX_SERVOS (_Nbr_16timers * SERVOS_PER_TIMER)
The key is 1 servo per timer. If you want to run 8 ESCs the max rate is 200hz with servos per timer at 2. Hope this helps someone.
Notably this will only work for the MEGA because it has four timers.