Never used it, never will.
I try to not use my Master Ball for anything in any PKMN game. I'm that awesome.
And I caught Zekrom in an Ultra Ball.
Got Kyurem in two Ultra Balls, can't say I'd recommend your plan.
I have more patience in my thumb than you. I did over 10,000 SR's for my shiny Virizion now stop making yourself look bad to other shiny hunters.I made sure I saved my Master Ball for getting a Shiny Kyurem, which I'm proud to have gotten after over 500 soft resets - and an insane amount of patience.