This is a great discussion.
However, unlike your CTX (FBS2), the FBS machines can't manage multiple targets under the coil at once, making the need for larger/smaller coils pretty much a necessity just as with an Xterra.
Your CTX is in fact a combination of both FBS and V-Flex technologies. A hybrid if you will, incorporating the digital aspects of V-Flex and multi-freq ability of FBS, which in turn is what enables it to do what it does.
So, back to FBS & V-Flex...
I hunt sites with both technologies, so I can honestly say that when a 6" coil is called for, regardless of which of the two technologies in this discussion is being utilized, an 8"-9"-10.5" or 11" coil isn't going to do.
I recently worked a homesite that was so littered with iron trash of all sorts that an 8" coil on my Explorer was a total waste of time. A 6" (which I have for the 705) was the largest coil that could be used if any level of sanity was to be maintained, and even with that and the faster response of the 705 I had to go snail speed to uncover anything worthwhile. In iron trash density like that, the fixed 3kHz 6" on the Xterra works extremely well looking for high conductors. It is capable of distinguishing good high conductors from rusty square nails and foil trash w/o the mental fatigue far better than an FBS unit. An unusual situation...absolutely....but we do encounter such.