The European CSDs Association (ECSDA) has published a very clear analysis of what happened when the European markets moved to T+2 settlement. The short answer is: Nothing. In most markets settlement performance continued unchanged or even improved during over the shortening of the settlement cycle. In only one out of the 17 markets did settlement performance deteriorate slightly but that was within the normal range of fluctuations.
Now attention turns to the US, where DTCC has just launched a new website to lead the move to T+2 settlement there.