Last year, fangraphs rankings had the Cubs' bullpen as the worst in the majors, and that's where it ended up. Of course, it had the Orioles as #29, and they made the playoffs largely on their performance in close games where the bullpen is most important.
Fangraphs also had the Cubs rotation at #24 last year, and they ended up 23rd in team ERA. Of course, they had Samardzija pegged as a disaster and didn't have half the bad starters factored in.
If the Cubs bullpen ends up with a 2.4 WAR as they have projected, that's roughly a 4 win improvement over last year just in the pen (-1.5 WAR in 2012). And the actual impact is probably a little more than that because WAR is context-independent. It doesn't consider that the bullpen is typically involved in high leverage situations. It's likely that an improvement to the Fangraphs projection would actually be worth 5-7 wins.