If I wanted to make my weightless superfluke or senko fall a bit faster using florocarbon, which lb test should I use? 12lb or 15lb? 12lb because it's thinner with less resisitance or 15lb because it'a heavier and a little stronger? I've baffled myself with question and figure I'd ask the experts here!

I use 12lb. Bass pro XPS and it works great.

12 and 14 of the same line will have the same density and will fall at the same rate.diameter only affects cranking due to line drag in the water.so a thinner line will have less drag and go deeper.

You'll have better luck using one size larger hook or going to hvy wire from regular, than you will trying to use fluoro as a sink tip.  If you need more depth than that, use a "Duper Fluke" from ***, it sinks faster than a Zoom.