I was wondering a question similar to this myself. I do have an initial answer but I would like to take the question farther.

As I understand how it works the ESC will add whatever timing you set above any of the mechanical timing on the can of the motor. So if you have your can at 15 degrees and you ask the ESC for an additional 20 degrees at max boost you are pushing a total of 35 degrees at max boost.

My question is for those racers that use spec class motors in MOD class; is it better to set the mechanical timing according to what you would run in Blinky and then add a small amount of boost timing on top of that...

OR reduce the mechanical timing on the motor to take advantage of more lower end torque then add more ESC boost timing for higher end RPM for the straights?

Example:
25 degrees on motor (set for Blinky) + 15 degrees Boost Starting @10K RPM for a total of 40 degrees at max Boost level
VS
0 degrees on motor + 40 degrees boost Starting @ 10K RPM for a total of 40 degrees at max Boost