you might want to re-post this on the FCP X Techniques forum.
this one is specifically for FCP& and under.
from my FCP7 perspective (and never having worked with this new full gate), however, i;d say that the full gate format would be difficult to work with as it’s a non-standard frame size (1:1.55 ratio)
maybe X can handle that, FCP would not be too happy,
and it would also mean working with a constant motion effect (and possibly a mask) on everything if you want to see the desired frame.
in FCP7, i’d be looking at making a 16/9 version, and adding a mask if it’s to be 2.40.
that give a decent amount of flexibility for re-framing
as for transcode format, that’d be a discussion to have with your editor and facility (or whoever is paying the storage).
i’d say that Proxy sucks. looks noticeably bad.
LT is a very happy medium> Looks good, with low data rate.
it’s what i prefer to use for off-line editing, and i do a lot of feature work.
does X offer LT?
nick