probably already old thread but i'm new to APM
and I had this same issue with NAZA v1:
when i fly in alt-hold, and i push the pitch stick forward to almos the max the quad fly fast but it loses altitude like 1-2 meters, maybe more, but when i stop it, the quad gain altitude very fast, like it was taking a breath :)
and then it stabilizes
my point is, ti seems impossible to do aerial photography following a car for instance, if the altitude drops right?