That isnt a good idea^^.
Magearmor counts as "leather" so you arent able to gain above 80%, maybe by the GGS.
To reach GM stealth or above you have to wear GM studded leatherpieces and add a GM bascinet.
every studded armorpiece you add, increase the difficult, best is you start with full medable leather at 80% and add at first studded gorget, then add glooves too, then sleeves, legs and tunic, at last put on the bascinet.
also u need high dex and try crowded areas like Vesper-Inn west from Vesper where many NPC around.
thats the only way to reach GM or above.