I've figured out that they've got 115 units to introduce, and giving them 1.5 years means that they have to average 1.5 units a week into service. They've already got a few ready and waiting to go, and I suspect that once the first one is in service, the rate of introduction should increase, rather than decrease. It's certainly not impossible, but I think you'd be right to say that its optimistic.
1. Final units are required for Dec 18 timetable change so almost 30 months exactly (Dec'17 (Jan '18), May '18, and Dec '18 are the key timetable changes). They'll need about 80-85 for Dec'17 timetable change so a delivery rate of 0.9units/week given (2.).
2. TSGN have 15 units already (may be another recent delivery?)
3. There are sidings and the test track in Germany full of units
4. Siemens can complete about a unit week and have enough translator wagons to enable more deliveries than that.
5. 8 units is about the most TSGN can usefully use for testing and driver training (assuming no overlap between the 2).
6. Pre-Hornsey opening, GN driver training and ECML link opening there is a limit to how many can be delivered without existing stock being cascaded/going ex lease (especially with 387/442 delivery/withdrawal in parallel). And there will be another 29x 387 to find space for temporarily as well before other stock is cascaded.
7. no point in delivering units at this stage if you know they are going to need mods later, far better to keep in Germany
Given the first unit has been complete for nearly 2 years and the stock already built I think the delivery rate will hit the required rate after they the first units enter service.