This is a difficult question to answer because no one here (I think) has ever been blind, so seeing for the first time is not an experience we can remember, we don't know what a person can feel when seeing for the first time.

according to a co-author of the study, Yuri Ostrovsky, a postdoctoral fellow at M.I.T, said that improvement was rapid and one child was proficient in less than a week

I thought the same, as time pass people start to get use to the new sense and their skills improve.

If a blind person were suddenly able to see, would he be able to recognize by sight the shape of an object he previously knew only by touch?

My answer is that it would depend on the complexity of the object. So a ball for example would be easy to recognize.