What I'm looking at IS the so called tweener GS skis which come in 170, 175 and 182 for Rossi/Dynastar GS. Next up is regular FIS skis afaik. But there might exist specific U18 skis too?
From what I can see just reading on their sites, it doesn't look like abrupt steps but rather gradual. Basic construction sounds like samish, but without, partial or full metal layers. I guess different material too in some parts. The 165 for instance has a full layer of titanal, the 158 has a partial and the 151(?) none. Binding and plate sounds like the biggest difference, and will of course contribute a lot to sturdiness.
And it's correct that all manufacturers offer these type of skis, but starting length varies. 165 is the last JR ski length for Rossi/Dynastar (other lengts for other brands) and I'm just trying to find out how big of a step it will be to move to the 170 or 175 for this brand. I'll talk to coach, but I doubt he has specific knowledge in this particular brand/models.