I agree with Carl, 30 minute sessions means lots more riding time, but less groups means larger groups, which in turn means more traffic on the track. And 30 minutes is probably longer than many riders, (myself included), can ride safely.
By way of comparason, consider Absolute. On weekdays, (when attendance is typcially lower), Roy runs three groups with 20 minute sessions. Bit on weekends, (when attendance is a bit higher), Roy runs four groups with 15 minute sessions. I actually prefer the 15 minute sessions. I'm happy to sacrifice 5 mins/hour track time in exchange for smaller groups. If Roy went with three groups on the weekends, I'd only get an extra 5 min/hour track time, and there'd be significantly more bikes on the track. Imo, not worth it.