The changes to the control box and pod might not be as simple as suggested in the handbook, the reason I say this is because Adam Harper is the only person I know who provided a 24v c5 with the pod still functioning.
The c5alive 24v conversion did not use the pod or original control box at all, so I assume Chris (creator of c5alive) struggled with it. C5martin tried to keep the pod functional during the conversion and wasn't successful either.
By going the 16v route you will be entering uncharted waters - meaning your experiences will provide very useful info for the rest of us
The gearbox will probably cope with 16v provided it is used sympathetically. I've managed to coax 18months out of a standard c5 gearbox on my 24v c5 "Voodoo", the controller was set to minimum. Even 12v c5's can trash a gearbox if abused - it is the heat caused by running under constant load that causes the destruction. The cogs soften,deform and then break up. Hence the Dragon technology gearbox experiment