I might be wrong but i don't think there is a way to modify your cruise speed, however you can alter your engine speed to be anything up to 990, to do that open goods.ini and find the engine that comes as an addon for your ship, you are looking for the nickname of this engine, then open engine equip.ini and find your engine, change the linear dreag to 50 or something like that and you will go really fast

Add CRUISEING_SPEED = ? under CRUISE_ATTEN_MOD_RANGE = 8.000000 in the constants.ini file in the [EngineEquipConsts section replacing the question mark with the speed you want to go at and add ANOM_LIMITS_MAX_VELOCITY = ? under CRUISE DEFAULT_ANGULAR_DAMPING = 0.200000, 0.200000, 0.200000 in the constants.ini file in the [PhySysConsts section replacing the question mark with the speed you want to go at. You will be able to travel at any speed that you want though the bar that displays your current speed will always read 300 if you exceed that speed.

If you want to see something strange, set the cruise speed to above 1,000,000 and head away from the system star at top speed. At +30,000,000km from the star, your ship will undergo what i call severe structural integrity deterioration without blowing up. The effects start earlier but its not as noticable. I tried it with the Liberty dreadnought and i must say it was amusing to see it.