I would have to say that this would be a very disappointing goal. From what I read it's somewhere around 1%, but probably much less.

I spent probably 30 hours trying to get all 219 shield from normal pit of saron on my 4 shamans in season 8 (before I got 1800 of course). With a 16% drop rate, I should have seen 6 of those shields drop, but only saw 1.

Realizing how bad your odds can be when you have 5 toons running together for a random spawn makes me have no desire what so ever to aim for this rare spawned weapon. It will be out of date in far too little time for me to have any desire to waste 40-100 hours on it instead of just farming for other gear/money.