I pulled the incandescent bulbs from my coin slots, and put red LED's. They look much better. I also put a super F'ing bright red LED under the normally white center "kill" insert. That insert only flashes for a sec when the ball goes down the inlane, and those 2 targets are a violent "sucker shot". I think it has good effect. Also this isn't really a "mod" but I pulled off the factory mylar, put down some carnuba wax, set the pitch high, and rebuilt the lower flippers, and wow is this game fast. Some guy was telling me, "I've played several brand new F-14s back in the day, and none of them played nearly this fast!" This guy does set all his machines up flat, with approximately 0 degrees of pitch, though.
I'm going to order a couple #89 sockets and add those 2 flashers, that seems like a good mod.