Have you seen any methodology work consistently when dealing with fixed deadlines? The reality when dealing with large contracts, as you mention, is that there are almost always timetables with expectations. This poses an inherant problem due to the unreliability of estimates, so either quality or features must be sacrificed if the timeline is in jeopardy. My only experience in such an environment was using some hybrid waterfall/sprint methodology that mostly worked fine, but i suspect that was more to with the general culture and people involved than anything else.