  We revisit a supersymmetric string model for space-time foam, in which
bosonic open-string states, such as photons, can possess
quantum-gravity-induced velocity fluctuations in vacuum. We argue that the
suggestion of light speed variation with lower bound from gamma-ray burst
photon time delays can serve as a support for this string-inspired framework,
through connecting the experimental finding with model predictions. We also
derive the value of the effective quantum-gravity mass in this framework, and
give a qualitative study on the model-dependent coefficients. Constraints from
birefringent effects and/or photon decays, including the novel $\gamma$-decay
constraint obtained here from the latest Tibet AS$\gamma$ near-PeV photon, are
also found to be consistent with predictions in such a quantum-gravity scheme.
Future observation that can testify further the theory is suggested.
