You should nearly guarantee it at turn 9.
Well, icky cases like this pop up and push you out to turn 10: http://dominion.isotropic.org/gamelog/201108/22/game-20110822-142645-6f762582.html
You misplayed it. Should have just gone ahead and trashed all three coppers on turn 6, and not bought anything. Turn 7, you still would have been able to trash a Copper with the Bishop and buy your second Silver that turn. You'd have been on-track for turn 9 at that point.
And that was all guaranteed. As of turn 6, your deck was a Bishop, a Chapel, a Silver, and 5 Coppers. After you trashed three Coppers, it'd be Bishop, Chapel, Silver, 2 Coppers. There's no way for that deck not to be able to trash a Copper and buy a Silver on turn 7. Then you'd have gone into Turn 8 with a deck that was Bishop, Chapel, Silver, Silver, Copper -- five cards, so obviously you could trash Chapel and go into the combo.
But the conflict doesn't hurt except to torpedo your chances for turn 8 (which you can't get anyway with Chapel/nothing). You still make turn 9 so long as Chapel doesn't miss a shuffle.
You can miss a turn 9 without Chapel ever missing a shuffle (I can show you how, if you like, but it's kind of complicated and I don't know that it proves much), just by drawing enough "keeper" cards with it every time. I don't know how likely that is, but it's more likely than with Chapel/Nothing, which guarantees you the ability to trash 4 cards with Chapel if you draw it turn 3 or 4.
Example of a deck play that would miss turn 9 with Bishop/Chapel, without Chapel ever missing a shuffle:
Turn 1: $4: Bishop
Turn 2: $3: Chapel
Turn 3: Bishop/Chapel/Estate/Cx2: trash Estate/Cx2 (total opening cards left: 5 Coppers, 2 Estates)
Turn 4: Cx3/Ex2: Buy Silver
Turn 5: Draw C/C, Reshuffle deck consisting of: (Cx3, Ex2, Silver, Bishop, Chapel). Draw: Silver, Bishop, Chapel. Chapel 2xC
Turn 6: Cx3/Ex2: Buy Silver
Reshuffle deck consisting of Cx3, Ex2, Silver x2, Bishop, Chapel
Turn 7: Draw: Silverx2, Bishop, Chapel, Estate. Bishop Estate.
Turn 8: Draw Cx3, E, reshuffle deck consisting of (Silver x2, Bishop, Chapel). Draw Silver. Do nothing.
Turn 9: Draw Bishop, Chapel, Silver, reshuffle deck consisting of (Silver, Cx3, E). Draw C, E. Trash C, E.
Turn 10: Draw Cx2, reshuffle deck consisting of (Silver x2, Bishop, Chapel). Draw Chapel, Silver, Silver. Trash C.
Turn 11: Finally have deck of Bishop/Chapel/Silver x2/Copper, enter combo. Buy first Province turn 12.
That's obviously ridiculously unlucky, conflicting tons of keeper cards every time. And it's also very, very slow. But I don't know how much the various combinations of conflicts that take you past turn 9 are. They're more likely than with Chapel/Nothing.
I guess I prioritize a higher chance of hitting turn 9 and lower chance of going turn 10 or later higher than a minority chance of hitting turn 8. I'm not
sure that Chapel/Nothing reduces your odds of going turn 10 or later, but my intuition is that it does.
Of course, all of this analysis ignores opponents using Bishop, which is an interesting case. It probably makes Bishop/Chapel better, since you may "make up" a lost trash off your opponent's turn even if you have a conflict.
