How far can a combat ops soldier improve?

Ever since the option came available when I unlocked the Combat Platforms I've been running around as a custom soldier by the name of Amber Fox.

Recently I managed to win my first S-ranked mission complete, and I immediately got a message that her proficiencies had "increased significantly" as well as a distinguished service medal.

When I looked at her profile again, all her skills had indeed upgraded from mostly C-ranks to straight As.

Now my question is: Is there an upper limit to how far a soldier can be improved in this way? IE, if I keep at it long enough, would she promote to A+ or even S ranks, or will I hit a ceiling and would be better off taking another soldier on missions to rank that one up as well?

(Note: This is somewhat related to the question "How do I make soldiers better?", but the only non-deleted answer it has does not answer my question at all -- and in fact it barely answers the question asked, but that's another matter).