Compare and contrast the BB84 and SARG04 Quantum Key Distribution protocols specifically in the context of a Photon Number Splitting (PNS) attack by an Eve with unlimited computational power but constrained only by the laws of quantum mechanics. Derive the mathematical upper bound for the information Eve can gain in the SARG04 protocol when using weak coherent pulses. Propose a decoy-state protocol modification to further mitigate the PNS attack and calculate the expected improvement in the secure key rate over a 50km optical fiber channel.