We've had Billy bookcases for over 10 years and they've even survived a house move without any probs. Our current house has high ceilings so I've stacked two extension units on top of the 202cm bookcases. The Ikea catalogues and displays usually just have the one extension but it is quite easy to use more than that. The top of each has holes for the little inserts for positioning the extension. We've attached the bookshelf as well as each extension to the wall so it's all pretty sturdy.
Judie one thing you could do to get the bookcases to 140cm (actually 141cm) is buy the smaller ones (106cm) and then put an extension on (35cm).