I think the quality of the thread implementation has a role to play. When a process waits on IO the OS will find something else to do. I don't know if user-level threads can be that smart, and I don't know if PLT threads are that smart.
Without casting aspersions, it's not just about number of processors. Very few programs avoid IO altogether, and if there's IO some programs will be more clever than others in moving work around.
In a properly working thread system, all threads that aren't waiting for something should be given a fair slice of the available CPU time. In particular, no thread that is ready to run should be blocked or delayed because some other thread is waiting on I/O.
If you are wondering if PLT threads work correctly, isn't this rather easy to test? Do an A/B test with some threads that do some computation with and without some other threads waiting on I/O and see if the runtimes are the same.
