Ok, answer is quite simple.Current string rendering on athens does not support kerning. Because it is not cairo's job. Kerning should be took into account after creating glyphs with cairo but before��extent measurement and obviously rendering. It can be done with FreeType.
I made small refactoring of text measurement and added kerning support.��
So, in the latest version of Athens-Cairo you can get extents easily:
extents := CairoFontMetricsProvider new
font: StandardFonts defaultFont;
extentsOf: 'Ta ffl'.
extent is an instance of CairoTextExtents.
Rendering on athens with Times New Roman size 200 gives: (Mac OSX Yosemite)
And here is what I get in Keynote with the same font
Close enough :D