I suppose I am an early majority adopter of scrum. After a couple of years working with it my ideas about what makes it work and it's deficiencies have firmed up, It's an excellent model for managing a team. It's not for everyone. The best way I have been to scale it is with well implemented project management techniques for example the WBS.
One of the best things about scrum is that it quickly becomes apparent what the bottlenecks are. The downside (depending on who you are) is that these are likely either team members or the project owners.
For many of us these are incredibly hard obstacles to overcome. Which is the harder for you will depend on your circumstances.
Sponsors, in my book are the easier to deal with. Not because the solution is easy to implement, but because there is only one real way to deal with the problem. Build trust through a personal relationship.
Poor performers on the other hand... Many project managers do not have hire/fire authority. The pathway to solving this problem lies in clear reporting of the consequences of no action. This can go both ways - up line to the sponsor and down to the poor performer. In some places there may be little or no consequence of poor performance.
Now, on the topic of aiming and firing - some cannon work;