The Scharnhorst effect is, at best, difficult to measure, and deemed unlikely to cause superluminal propogation. To quote John Baez:
"However, further theoretical investigations have shown that once again there is no possibility of FTL communication using this [Scharnhorst] effect."
Several papers disputing the Scharnhorst result were published circa 1990, unfortunately, finding free sources of these papers has proven difficult.
Quantum Noise and Superluminal Propagation
Regarding slips in the arrival time of high energy GRB photons:
Testing models for quantum gravity
Selected topics in Planck-scale physics