Setting QFont::stretch to anything other than 100% doesn't work properly on Lion. The glyphs change width, but the relative spacing of the glyphs doesn't change.
I suspect this is a bug (or perhaps a behaviour change) in the OS, but it seems as though Qt needs to take it into account.
The problem is reproducable with any stretched text; for a simple test, try running the test app I attached to
QTBUG-22652 and switching on the 'stretched' checkbox on Mac.