0.047" I believe. I'm just throwing this number out there because it was the first thing that came to my mind. DON'T GO AND BUY THAT SIZE AS SOON AS YOU READ THIS! LET ROB CONFIRM IT FIRST! lol I'm not 100% sure, it's just a guess.
I haven't worn out my metal diff yet. I run it hard and it continues to hold up. I would put my money down that it will hold up to a 4-5S HARD run and do fine. You have nothing to worry about, except keeping enough grease in the gearbox.
Does your diff have those little shims behind the bevel gears?