That is a complicated question: 1) Single page load: no. This is generally an IO-bound kind of system, e.g. an infinitely fast processor would have almost no effect on response time.
2) Concurrency/Scaling: generally, no. This is dominated by the DB backend, and (generally) the speed at which the DB can serve concurrent queries becomes limiting well before cPython gets too slow. 3) Any code that gets "exec"ed by python: no. AFAIK PyPy doesn't do anything to exec() code, and that pretty much means your whole app. (I had a look---I can't find this mentioned anywhere, but I seem to have this idea stuck in my head from somewhere. Perhaps it was a limitation of an earlier PyPy version and no longer applies?) What remains is the internal stuff, such as markmin, templating and other gluon/* code (and rocket!). So the final answer is a tentative "no", PyPy probably won't have too much impact on performance for web2py. So why bother? Because PyPy is the future of Python.
