we've come to the conclusion that this does NOT work for upping Trojan armors, Archer coats, and Warrior armors to 120. Over 80 mets put into an elite+4 1 soc archer coat and they all failed. Over 50 put into a Super+5 1 soc trojan armor, all failed...
HOWEVER, to further support this, my wife upgraded her robe (super+4) from 100->115 in 7 mets. Since I began using this trick, it has not cost me more than 6 mets to up ANY of my own gear, and anything not being upped to 120 has worked just fine as well (except for 1 friends super+3 earrings, which refused to up from 97->112....)