Well, a current model GS ski at 183 cm is more stable than a 208 cm GS ski from about 20 years ago, so there must be a little bit more to it than just length.
Specific to your question though, how about a 17-year old J2 racer who is skiing on a K2-targeted GS ski, e.g. 165 cm & 18.5 m? A decent J racer will out-ski this equipment fairly easily.
However, a 14-year old K2 racer, maybe 35 lbs. lighter, will manage very well with that ski, and might even beat the J2 racer in the same course, even when the J racer gets on a more appropriate 23m FIS ski.
How can this work? A given ski has the capacity to manage a certain amount of energy, and if you exceed the limit (or can't reach the target range), results are compromised. F=ma again, and there comes a point where more is not always better.
There was more to it than length even 20 years ago. A 208 Kästle SG was more stable than any >208 GS ski I was able to demo. I think it has to do with the skis ability to dampen oscilations, a trait in which total mass has a part to play, both in how much mass needs to be stopped from oscilating and in how much mass is available to stop other parts of the ski from oscilating. The 215 Kästle SG was more stable than the 208 SG, in part because it had more mass. I think the amount of rubber and other energy absorbing materials and the strength of the ski in transmitting that energy to other parts of the ski plays a bigger role.
If a ski has the sufficient structural integrity and energy absorbing material, then the longer ski will be more stable at high speeds and have a higher ultimate speed limit. If the ski doesn't, then the longer ski will be less stable. While a well-designed SG ski is more stable at a greater length, take a softer gentler ski like an old Rossi Bandit and crank it up to 60 mph, longer than 176 will be worse, because you will just have more ski flapping around to wrangle.
As to the J2 racers above. The too-light for the ski skier will have a worse time on the longer ski because he isn't producing the forces to make the ski work, and the heavier skier will have a worse time on the shorter ski because he is overpowering it. True enough, but I don't think either skier would reach either ski's speed limit on a GS course unless he skipped the gates and straight-lined the run. You can easily overpower a ski at speeds well below the ski's stability speed limit.
You could be onto something with the energy input though. Let's explore skier mass as a factor in putting energy into the ski straight-lining or nearly straight-lining it down an icy run at 60 mph.