I prefer use [ExecuteInEditMode] instead of overriding defaults because you can edit it realtime in editor and you can grouping all texts , then set defaults to different groups of them.I know you want pervent repetitious work(changing defaults) by overriding defaults but you can't grouping them. for example:

group1(Menu fonts features) : font = 15;fontstyle = Normal,.....

group2(Score board fonts features) : font = 20;fontstyle = Bold ,.....

group3(Button fonts features) : font = 10;fontstyle = Italic,.....

but If you override them you have to change default again for some text,so you're limit in this way.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

[ExecuteInEditMode]
public class ChangeFontSize : MonoBehaviour {
    public enum FontStyles{Normal,Bold,Italic,BoldAndItalic};
    public FontStyles ActiveState = FontStyles.Normal;
    public Color color;
    public Text[] AllText;
    public bool ChangeDefault;
    [Range(0,15)]
    public int size;

    void Update(){
        AllText = Object.FindObjectsOfType (typeof(Text)) as Text[];
        if(ChangeDefault == true){
            foreach (Text txt in AllText) {
            txt.fontSize = size;
                FStyle (txt);
            }
        }
    }
    //pass text components here
    void FStyle(Text mytext){
        switch(ActiveState) 
        { 
        // Check one case
        case FontStyles.Normal: 
            //Set Normal Font style
            mytext.fontStyle = FontStyle.Normal;
            break; 
        case FontStyles.Bold: 
            //Set Bold Font style
            mytext.fontStyle = FontStyle.Bold;
            break; 
        case FontStyles.Italic: 
            //Set Italic Font style
            mytext.fontStyle = FontStyle.Italic;
            break; 
        case FontStyles.BoldAndItalic: 
            //Set BoldAndItalic Font style
            mytext.fontStyle = FontStyle.BoldAndItalic;
            break; 
        }
    }
}