PJR suggests 5.2 oz/sq yd or 177 gsm for this size of sail, when conventional sailcloth is used. This is plenty strong enough for the whole sail.
I have found that the cloth breaks down in three main areas:
1. At the throat of the sail, due to cyclical loadings, first on one diagonal, then on the other. The answer is not heavier cloth, but patches that put the threadlines at an angle to the panels. Sailcloth can withstand a pull along the line of the threads, but is not so good at withstanding diagonal pulls.
2. Down the leech, fluttering and flapping will cause breakdown. This happens more in the lower two panels than in the upper panels, because there is little or no tension in the cloth. The answer is to add a doubler all down the leech, about 300mm wide, if you want to make an ocean-going sail. Again, there is no point in using over-heavy cloth throughout the whole panel, and for cambered panels, lighter cloth will take shape better in light airs.
3. Threadlines should be close to parallel or perpendicular to the edge of the sail, where the loads are concentrated, or the cloth gets distorted and will break down.
I have never seen cloth breakdown in the top panels, once the peak and throat have been reinforced with patches, in the conventional sailmaker's fashion. So I don't think there's any need to use heavier cloth here.