I've noticed something about the IGT recently, and will be keeping it here for some documentation and future runners.
When the ending cutscene begins after the final switch/portal is reached in a level, generally it takes 2-2.3 seconds on the in game timer for the level to actually end.
If you pause as the final point in the level is reached, and add 2 seconds on to the timer in the pause menu, this will generally give you an idea of the time you will have for the level.
This is just an easier way to determin if you'll have a PB for the level. Instead of praying the IGT is lower, having to return to the main menu to type in codes, or run the whole game again up to the level you are attempting to get a lower time in.
Example: finish level 1, pause as switch is pressed