LOL! I sort of figured that's what you wanted to know; I had all the same questions when I started researching fEARfuls. As a disclaimer, I'm not too qualified to answer most of the questions b/c I don't own one yet. The kit is literally in the mail as I type.
But from the research I've done and reading all the fEARful threads about a hundred times I've learned a lot. First off, the main magic with all of the fEARfuls (there are multiple 12" and 15" variants) is that they are designed with no compromises. There's enough internal volume, enough port area, cab dimensions are what they are to prevent standing waves, cab is well-braced, crossover uses beefy parts, etc. The cab is matched to the drivers, the crossover is matched to the drivers. No "generic one size fits all anything here.
The 3015LF and 3012LF have high excursion capability and can move massive amounts of air. The spec'd 18Sound mid driver is actually a pro audio PA cab driver and handles a lot of juice. Big output drivers crossed over at their optimal frequencies in optimal enclosures = big, clean, balanced sound.
I know that a 15/6 easily slays any 410 in output and doesn't "beam" like a 410 does. I'll have to defer to the rest of the TB members for actual fEARful vs. Cab X comparisons.
Hope this helped a little bit.