Both really good questions. That was started almost two years ago so we've worked on a variety of different types of projects since that initial study--from simple websites to high-availability, rich applications. Some have required more internal knowledge of Django than others, but I can't think of a time where Django itself was an impediment or a bottleneck.
With regards to time series...me too! I don't have enough data to make any statistically-backed claims, but anecdotally Django-based sprints do start off with a big bump in velocity, because you're handed a tremendous amount of functionality upfront via the framework itself, Admin CRUD, etc.. Once a team gets moving their velocities tend to stabilize, which is one of the reasons story points and velocities are such powerful estimation and forecasting tools.
That being said, C# teams seem to have larger standard deviations around their mean velocity. One thing I have noticed is that refactors tend to have differing impacts. Major refactor efforts and rearchitecting almost always noticeably hit the C# team's velocity much more so than the impact on the Python group. Having done refactoring and and rearchitecting in both environments, I tend to think it's a function of the syntactical overhead associated with the C# language.