This is a great idea.
If adding to the gcodes:
The added file size should be a non-issue. At most, you might add an extra 5-10% to smaller files, maybe? And even less on larger prints.
If outputting your settings:
The file size should be relatively small. The added benefit to see exactly how the print was set up, could aid in narrowing down the particular issue plaguing the user. We could see temps, layer heights, fan speeds, etc.
Adding to the idea:
I would think adding a section for user-inputted notes would be a good idea. We could include in this section our material type and manufacturer, as well as other things we may think are important.
This would be a great tool for troubleshooting!