The yellow and red Gottlieb flipper coils are interchangeable, physical size is almost the same between the two. Wire gauge and amounts of windings determine strength and hold inside the coil. Heat has nothing to do with coil strength per say but can weaken or damage a coil due to extra resistance as it overheats. If a flipper coil is getting hot, something else is usually contributing to it.
The problem I see is that the red coil is a lot stronger and can break parts like drop targets or plastic sets that are too close. If you look at most manuals, Gottlieb didn't use red coils near drop targets already because of this. If anyone choses to go to a stronger flipper coil, they do it at their own risk since some plastic parts can't be easily replaced. I'd rather rebuild a flipper assembly than replace hard to find parts.
Another trick to get more power out of the flippers and kickers on some of the System 80 games is to add another 10,000 mfd 40VDC capacitor to the 24VDC bridge rectifier. I don't advise this for long term but it's fun to see how strong one change will make the game. Some system 80 games that split the controlled (24VDC) and uncontrolled coils already have this cap installed (38VDC) from the factory (HH comes to mind here).
System 80, not just a job, it's an adventure