If you mean after pressing "Ctrl-I", I'm not sure if that can be changed. looks more like it's hardcoded to a default value.
if you want to increase the "Engage Warp" mod speed, you simply need to change the ship's default speed.
Intercept Acceleration also depends on the ship's default speed. it just re-scales your impulse settings so 1-8 are the previous 1-9 settings and 9 is like "Engage warp"
to change a ship's speed, use notepad and open the ship's hardpoint (scripts\ships\hardpoints\whatever.py) and search ".SetMaxSpeed". to give you an idea, 10 is about 6000 kph, so the normal values are between 9 and 12.
play around with accelerations, speeds, and masses so you get the effect of a light fast ship like the Defiant or a big slow and heavy ship like the Galaxy.
instead of notepad, I suggest MPE. much better when it comes to open .pyc