My gymmie was good as a L5 and even though her placements at times in L6 weren't as good, I thought she became a better competitor and gymnast with that additional year of compulsory(we don't compete L4). L6 is tougher than L5---no doubt about that. The judges want those skills bigger, stronger and with confidence. I remember at the 1st L6 meet one of the other moms leaned over and said, "where did all those 9s from Level 5 go?" I think bars is the biggest challenge and doing that year of really working on good clear hips, "baby giants", casting to handstand(not required) and really getting the feeling of "swinging" bars helped her be very successful as a L7. Beam can be a little bit of a challenge also with the bwo, full turn and more advanced jump combo.
JMO, but I think L6 really sets them up nicely for moving onto optionals and as it is said around here alot----its not a sprint, its a marathon.