The Dark Knight Rises did 30.6M (32M adjusted) in 2012 with midnight showings only, lower ticket prices, no 3D and a lower IMAX share than AOU. Granted it was in July.
27.6M for previews is better than FF7 and better than TWS but as far as I recall none of these films opened above 200M which is the benchmark for AOU.
Now what people should really understand at this point is that the movie is going to be a lot more front loaded than its predecessor and that it IS going to impact the opening week end (internal multiplier and all). Meaning that the bigger it opens the higher his chances to reach some important milestones, ow record first and then 500M, 550M, 600M dom.
27.6M for the previews is on the lower ends of the predictions/estimates, let's hope things are gonne get better from now on and the the fight on Saturday won't make a significant dent to the ow gross because let's face it, Marvel would have every reason to be at least slightly disappointed if the film doesn't break that ow record. Gangbuster overall business or not, that would make for a sweet headline.