Over 1000 games on my Met premium without any airball protector and no issues at all, so it's not a must. I not saying don't buy it, just that it is not an essential item as many on here like to insist.
For me, the #1 mod has to be the Color DMD. I have it on several of my games but none look as good as Met. I was also blown away by how good the speaker light kit looks. It is sweet. I will also echo that the snake tongue and fangs are a must because you want a snake, not a frog! I did swap the metal block for a real captive ball. Seems like a no brainer because it looks better and makes it a better shot.
Also, take those damn spot lights off the slingshots and relocate them to the Fuel targets and the scoop. The slings look so much better without them blocking the view.