A hard one to answer certainly (just look at all the replies).

It might be your pre-bend, but it might also be battens, the way the sail is cut, your sheeting/downhaul/helming etc. Going out trapeezing will also change how the mast bends, so there are lots of variables. If you were sailing uni with a sloop mainsail, I would expect the lower telltale to stall first.
Changing pre-bend will not only adjust the leech, but draft position and amount. Only way to be certain is to change it, and then race an identical boat to see if you now are faster or slower (or have better/worse windward abilities/height).
The sailmaker who cut your sail should be able to tell you what amount of pre-bend he cut the sail for. Usually a sensible place to start off when adjusting and trimming the mast.