Penn was smoking Matt Hughes before he gassed and got beat down, and he took GSP to his limit. Sherk has a similar style but is not close to the level of Matt Hughes. Just comparing their performances to common opponents in GSP and Hughes shows that Penn is a lot better. Sherk's only prayer is to hope Penn's cardio is weak, but against Stevenson it appeared to be better than it ever has been before.
Sherk is not going to LnP BJ. Penn is a former WW champion so Sherk's size advantage won't matter as much as it did against Franca.