What are the most effective ways to scale Extreme Programming for large projects?

Merely adopting the nested structure will not remove dependencies. There will be dependencies between teams, both in terms of communication and work. This can make dependencies even worse, because teams would have to wait for each other's delivery to be completed, in order to pick their items. This introduces high lead times. In those cases its important to follow some of the Kent Beck's advice to introduce periodical shared planning or temporary squads which join two teams together to accomplish user feature.

What can be useful is the concept of information radiators which is native to other Agile frameworks. Information radiators are displays or other "screaming" means of communicating what the team is currently working on. This way teams can be better aligned.

In large projects, the influence of a single coach or a few facilitators can be limited, making it challenging to ensure consistent application of XP principles across all teams. It would be better to have multiple team leaders who are well-versed in XP practices across different teams. These leaders would ensure that XP principles are consistently applied within their teams, while coordinating with other leaders to maintain alignment across the project. This approach would help solve communication and coordination challenges in large-scale projects more effectively.

It is important that a coach or facilitator needs to have practical engineering knowledge in terms of XP. It needs to be a person who lived and breathed the XP by experience as those practises cannot be learned properly by just reading some theory.

It always amuses me that we use the term "scaling up" without thinking of the obvious analogy of a kettle. When your kettle "scales up" it becomes inefficient and broken. Why we believe any differently for software delivery is a mystery. Instead of scaling effective processes up to a larger size, break the work down smaller. Years of research demonstrate that project size is one of the biggest predictors of failure. Aim for small autonomous teams working in small batches and you can avoid "scaling up" and breaking.

Provide agile coaching and facilitation to support teams in adopting and adapting XP practices effectively. Use appropriate metrics and reporting tools to track progress, measure value delivery, and identify areas for improvement. Encourage experimentation, innovation, and learning within teams. This allows them to adapt to changing circumstances and find new solutions for scaling challenges. Invest in training, development, and tools necessary to equip teams with the skills and resources needed to scale effectively. Recognize that scaling XP requires flexibility and adaptation. Be prepared to adjust your approach based on the specific needs and challenges of your project.