I don't know if the suptitle() is implemented via OffsetBoxes, but the What's New text mentioning this:

When tight_layout() or Figure.tight_layout() or GridSpec.tight_layout() is called, OffsetBoxes that are anchored outside the axes will not get chopped out. The OffsetBoxes will also not get overlapped by other axes in case of multiple subplots.

made me hope that I could finally use fig.suptitle() with ax.title and fig.tight_layout() without manually adapting the margins. Alas, this still seems to be an issue:

Version: '1.5.0rc3+207.g54ead93' on OSX, self compiled using python setup.py install with conda's Python 3.5

Code:

fig, ax = plt.subplots()
fig.suptitle('suptitle')
ax.plot(range(10))
ax.set_title('title')
fig.tight_layout()