The Zoomers are good if you are swimming with people faster than you and you can't hold the send off time and you want to keep up. They give you just enough boost to plane you out in the water better and improve your speed. They are also excellent for the Total Immersion drills if you have a poor kick.
The third gear in your little auto analogy would likely be reserved for the foot pocket fins. For the hypoxic drills Zoomers are too short and don't give enough thrust, the Rockets and the free diving fins are too stiff and long and are cumbersome in the pool and very taxing on the legs for fast swimming. I have found the fins with the foot pockets allow for a nice fast kick with a short arc that keeps you from ruining your hydrodynamic profile. If you fin too big your feet and fins will create a lot of drag.
I always swim with the Seal goggles, very comfortable and great vision. Now while I am all for the $25 Seal mask, paying $130 for a snorkle seems awully high. The purge valve in the snorkle allows you to clear the water inside it by blowing it out the bottom which is easier than blowing it out the top since gravity helps rather than hinders your effort. I use a Finis freestyle center mounted swim snorkle for my drills...much better than a regular scuba snorkle for swimming. The metronome might be useful for keeping a low stroke count but I think the sound would annoy me. I like the underwater radio but haven't bothered to get one. My swim workouts take from an hour and 10 minutes to an hour and 30 mimutes so it would be nice to listen to something for all that time. Always more toys.