Interesting. In the case of the airframe, think the issue would be more G force than actual speed. That said, the 38mm and 54mm LOC tubes have more than enough compression strength to handle most anything you will throw at them.
In my mind, flutter is the first thing to create a "speed limit." If one or more fins flutter and partially or completely fails structurally, you can get high yaw angles, or at least oscillation, which loads the tube laterally. Then the strength of the cardboard becomes a limitation. Another factor is the length of the airframe. It always wants to be a giant fun noodle and bend. The relative bendability goes up as either L^3 or L^4, I forget at the moment. The fashion of making rockets longer than they need to be to fit all the propulsion, recovery gear and payload, presumably because it "looks zoomy," works against us here. (As well as adding skin friction and weight that slow everything down.)
I haven't settled in my mind the relative importance of g-force (acceleration) vs. flutter in the failure sequence. Flutter is not only a transonic phenomenon. It can happen at almost any speed if the dynamics are right (very flexible structures). It's how a woodwind instrument works. I think it's more likely that fin flutter leads to aerodynamic excursion leads to airframe failure. I'm less inclined to think that fins simply strip off the tube due to being "left behind" by the acceleration. But if a fin does strip, that leads to aerodynamic excursion leads to airframe failure.
Also, a really long, thin-wall tube is subject to bending failure due simply to the inevitable flexing that happens during flight. This has been written about extensively in the "SuperRoc" community, which is not generally dealing with anything like transonic phenomena. An Estes Star Orbiter is simply a better rocket in every way related to performance if it's built with only one of the tubes. Build it with the VanderBurn fin upgrade and one tube, and it will be massively more capable. Replace the tube with 38mm MMT, which is effectively the same diameter, and it will go supersonic no problem.
To get the most speed out of cardboard, make it thick enough, make it no longer than it has to be to fit everything, stuff as much of the heavy stuff (trackers and altimeters) as you can as far as you can get it into the nose for stability, and make sure you don't flutter the fins. Then make it smooth on the outside.
Hmm... I have a VB SO fin upgrade kit on the shelf. This is making me think about building a minimum-length BT-60 rocket with it just to see if the Estes cardboard will live supersonically. Maybe I'll cheat and put a coupler in the fin can, but leave the rest thin.
Note that maximum apogee and maximum velocity, even with the same motor, may point toward substantially different approaches to weight and its distribution.
