You can't. Actually this is not a "padding". getBBox returns dimentions given from a font metric, not individual glyphs. BBox height for text element includes font ascent and descent.
In most fonts, ascent reserves a gap above cap-height for glyphs such as "Ä". Discenders are reserved for lowercase characters with "tails" such as "g", "j", "q" and etc. For example, draw a rect around text "Äg".
For more detais see:
http://commons.oreilly.com/wiki/index.php/SVG_Essentials/Text
http://www.w3.org/TR/SVG11/text.html#BaselineAlignmentProperties
http://en.wikipedia.org/wiki/Baseline_(typography)