if you're looking to make it no longer work perfectly, adding more or less balls than it's designed to operate with is a darn good way to accomplish that.

If more power ball was the goal, why not just remove a steel ball and add a second power ball?

Game was originally designed for 7 balls - 6 steel, 1 powerball.But that's not what the 4th trough switch is for.
The game will appear to operate correctly, most of the time. But you'll run into issues like, for example, locking a powerball in the lock. Or the game thinking the powerball is in the gumball and trying to load magnets for multiball. At worst, you'll get some oddities and a credit dot.

I’m running the home ROMs. There’s a setting to add another ball to the gumball, so the game will operate properly with 7 balls—as designed.

Not sure I’d want to add a second powerball though. I like that it’s a rarity.

I have seven balls, home rom with the option mentioned so it works properly. But only 1 is a powerball. Makes power ball more rare which I don’t exactly like, but LITZ is a bit more exciting. Like you said it’s your game so 2 power balls sounds grand. I wonder if game might get confused in some cases though.

the 4th switch is to let the game know, the gumball machine needs more balls. Anything that hits that 4th switch, will launch another ball in the gumball machine. So if you put 15 balls in the game. 12 will be in the gumball machine. or try to be anyway.