gsteinb and rkwaki said essentially what I meant. Masters should never be forced to race with Cat5's unless they are Cat5's themselves. Out here we don't even call them Masters, they're Cat5 35+, 45+ or whatever. Most Masters races here are Cat1-4 only with a few Cat1-3 only. The only time I want to be in the same field as a Cat5 is if I am mentoring them as part of a clinic. Club/group rides don't count. Anyone can be an idiot on those.
What you folks are doing out there is dangerous. You are grouping racers with a wide variety of ability, from zero to world class, because of some perceived notion that they'll race at the same speed. That's putting the promoter's interests over safety.