Views on cruise extensions please?

I have booked on the Empires of the Mediterranean cruise from Venice to Istanbul in Sept 2015. Viking have only just released booking to Uk customers in the past few weeks. I was wondering if any of the contributors who may have booked this cruise are aware how the extension bookings are going. I am interested in the Italian Cities 3 night extension pre-cruise as well as the two night Istanbul extension post cruise but I have not yet booked them. The delay in booking is a mixture of a) to limit the amount paid so far in advance and b) I may possibly travel on the Orient Express from London but the train operator have not released their 2015 Timetable yet (no surprise!). I don't wish to leave it too late to book and find the extensions fully booked as I have read elsewhere for earlier cruises in this maiden season. All comments welcome.