awblain,
You are of course correct in your observation that my 'simple sum' is based on nothing else than the conversion of kinetic into potential energy. That is no more than a first approximation (an upper bound) of what can happen in this sort of scenario.
I probably misunderstood MrSnuggles when he wrote:
These are all ballistic calculations and as we know that is a huge amount of drag that are not taken into consideration when calculating ballistic scenarios.
I have not made any drag (non-ballistic) calculations ...