So what will it take for them to make the playoffs?
Looking at their history (http://www.baseball-reference.com/teams/CIN/) from 1995-2011, comparing runs scored differential vs. playoff appearnces (since 1995 was about the wildcard implementation)
1995 +124 1st place lost NLCS 0-4
1996 + 5 3rd place
1997 -113 3rd place
1998 - 10 4th place
1999 +154 2nd place lost WC play-in game
2000 + 60 2nd place
2001 -115 5th place
2002 - 65 3rd place
2003 -188 5th place
2004 -157 4th place
2005 - 79 5th place
2006 - 52 3rd place
2007 - 70 5th place
2008 - 96 5th place
2009 - 50 4th place
2010 +105 1st place lost NLDS 0-3
2011 + 15 3rd place
So it looks like they need to get about a +100 run differential for the season to make it into the playoffs.
Last year they scored 735 and gave up 720.
If you figure that Latos replaces Volquez/Willis, Volquez/WIllis gave up 114 runs. Latos gave up 82 runs. If you bump Latos 10% for the switch to GABP, that puts Latos at 90 runs. 90-114 is a reduction of 24 runs from last year.
Cordero & Madson had nearly identical ERA's, so no change there. Marshall is a mid 2.50 ERA guy, He will push everybody down the chain, so assume he replaces a 4.50 ERA. Over 75 innings that means a reduction of 17 runs for the season.
If you assume Cueto has his ERA rise a bit, becuase sustaining a 2.30 ERA for 2 seasons is hard, but if you assume Leake & Bailey get a little better those cancel out. If you assume Arroyo gets better and drops his ERA back to his 2008~2010 level, that's a reduction of another 15 runs.
Their runs scored probably don't change much from 2011. If anything it may get a little worse, so let's guess it drops from 735 to 730.
So 730-664 = 66 run differential. Good enough, historically, for 2nd place. If the Poo Holes & Fielder defections from the NL Central weaken the competition, then 66 runs might be good enough to win the division. Maybe.