Christ we're a negative bunch aren't we. Interesting question though.
Personally I'd expect Mancini to set the team up to get 50 points from the bottom half (thats dropping 10 points max), look at his Inter side they were a model of consistency. Then I have no idea how he would do against the top half; at times we looked clueless (Everton, Spurs), sometimes we looked able to kill a game well (Liverpool, United) but other times we looked immense (Villa, Brum, Chelsea).
I think it would all depend on whether he could win the dressing room over or not. If he could then 4th if not god knows how far we would slide.