If it increases your productivity up to 300%, that means that you can work for 5 months, then take a month off. You will still get ~30 months worth of work done in a year. So thats a huge success.
I solved problem by training team, which replaces me. Then I leaved my work (IMHO, crisis is best time for that). Team works slower than me. Their solutions are not so good, because nobody has picture of whole project in his head. However, they can continue to work, while I cannot. As I can see, this scenario is typical.
IMHO, problem with XP and SCRUM that they producing too much load on the few competent persons. Overall result is better than with traditional methods, but competent person burns much faster, because they contribute more than others.
It is hidden magic beyond the process - XP and SCRUM just delivers task to competent person much faster than any other process. The drawback is that there no reason to grow for other developers, thus load on competent persons tend to increase over time. Pair programming and peer reviewing helps when people are roughly equal, but that is rare.
If we try to model situation, then we will see few major stages in evolution of project and/or group:
* fast growing of project and/or group until physical limit of few key people will be hit;
* degradation of group, because key people leave project, until there no overloaded people;
* stagnation.