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I have made some comparative tests with OpenSans and ttfautohint 0.8. with WinGDI Cleartype and Greyscale.
Tested original font (OpenSans regular, Light, Extra Bold), plus - unhinted, ttfautohint with 'taller xheight', and regular ttfautohint versions.
The most significant issue remaining is the rendering at small text sizes with bolder weights, e.g. ttfautohinted versions can't yet emulate the finesse of a human manually hinting chars such as 'a', 'e', 'g' etc with such few pixels to play with. However, to my mind in the medium size text ranges i can see characters rendered a bit 'better' by ttfautohint ;)