Hello!
I was wondering, why dont you set spacingX propery from int->float
(Same in NguiText - remove RoundToInt: )
NGUIText.spacingY = Mathf.RoundToInt(mScale * mSpacingY);
With float properties it is possible create fancy text tween animations, like here: steamdefense.com (From main menu go to Options->Cast, when it is showing "Steam Defense" name)
Attaching TweenSpacing script