Code:
I've tried overwriting zo_strformat,
Not a good idea! These functions are used everywhere in every game and addon related string handling, so if you mess with this you will most probably destroy everything that relies on this function
Some parts of the tooltips use a constant like SI_QUALITY_TEXT etc.
Tooltip text is build before the tooltip opens and just replaced in the tooltip into the palceholder SI_whatever as far as I know.
Chekc other addons code like Alchemy Tooltips.
Could be that the stats are not handled the sme way and cannot be altered! But there was an addon "CritPercent" or similar which altered the text to show the crit percentage instead of values I think, so it might be possible.