So I've mostly written a simulator (by hand!) that just buys Silver or Sentry every turn and plays all the Sentry it can each turn. Before I run it 10,000 times or whatever to see how many cards it trashes per play, I just wanted a few opinions on constraints:
1. How many turns should I run per game? (Ten?)
2. How many Sentry to buy? (Two?)
3. Always topdeck Silver or always discard Silver? (Topdeck?)
So far early results seem to indicate it is rare to trash 2 cards with Sentry more than once.
I'd say ten turns should be plenty to look at. Barring sloggy games with lots of junking, you should be shifting from deck thinning to engine building by that point, and less concerned with trashing multiple cards. It seems for the purposes of this discussion, we're mostly interested in the early game trashing ability of Sentry.
You could try testing both one or two--I wonder if adding that second one really increases your chances of trashing two cards on play, my guess is that it doesn't.
I'd say topdeck silver, since especially in the early game, playing your silver for economy will usually be better than cycling it.
It would be interesting to see the difference in trashing efficiency between getting Sentry before the first shuffle vs. before the second.
I also like JW's idea to compare with Junk Dealer (e.g. on average, will two Sentries trash more or fewer junks than two JDs?).