I think the idea that it becomes better in multiples is valid. Punisher-type mechanics are usually bad but when an effect is stacked up it does become much better. It's in an awkward spot because I don't think you can squeeze in more than 1-2 copies of it without sacrificing another key part of the deck, and having only 1-2 copies won't show up very often, so testing won't be very conclusive. Honestly, go for it. With one medallion it becomes Shrapnel Blast, with two it becomes a miracle-triggered Thunderous Wrath. I think this is how you need to view it, because getting 3 cards is just too risky for any opponent unless they are at 5 or less life. In that sense, it gets better the later in the game it goes off.
I can't help but think that maindeck Guttersnipe would be the way to go if you test out more than 1-2 copies of Browbeat. At that point its more of a Pyromancer's Ascension/Bolt combo deck that doesn't really need storm count to win. Maindeck Grapeshots would also be a likely good move.
In the vein of weird cards to test out, I was thinking of this off-the wall tech:
Tangle Wire
It will have little to no effect on us (tapping one land) but with 4x SSG I could see us working around it. It allows us to get a few more draw steps in and halt progress from opponents while we set up. It doesn't do anything against Force of Will, but we don't have anything for that anyways. It will however protect us against stuff like Spell Pierce and the board state becoming overwhelming.
Thoughts?