G-LOC can occur at relatively low Gz. Variables such as pilot fitness, previous G-loadings (eg negative Gz just prior to positive Gz will need less positive force to produce grey/black-out), and most importantly length of time sustaining a particular load, will affect when a person G-LOCs. I'd have thought that the Red Bull pilots sustain the G-load for quite a short time, pre-stain their muscles (to slow the flow of blood away from the brain), and have light-weight flying kit (such as no helmet, goon bag, LSJ etc) which puts less of a strain on the neck and torso.