Here's the thing: Yeah, we have blown some leads in the 6th and 7th inning, but so has every other team. The other thing is that our record in games TIED in the 6th and 7th innings is actually quite good, and the bullpen has a large (positive, in this case) influence in those games.
As I linked above, bbref has the Braves' bullpen at a minus 1.8 games compared to the average National League bullpen. I think that is about right. And as Gilesfan has pointed out (shoot me for THAT) the true effect of middle relief on game outcomes is pretty marginal.
That said, I would like to have a couple of extra wins now, and middle relief IS something that can be improved (maybe it already has been, if you look at recent games) cheaply.
To answer your question, though, --two extra wins is about right, but not four, six, or eight as some have postulated.