I'm with Dick on this one. From having flown some light weight "racy" high speed models as well as light slow speed model in all sorts of conditions I never had any issue with them. Of all the planes I've flown by far the easiest to fly in the worst conditions was a small lightweight 1/2A racing model. It flew so good I actually built two of them. The speed allowed them to slice right through turbulence with no noticable effect other than slight displacements in roll or a sort of flat sideways or up and down kick here and there. They never suffered from the sort of pitch related disturbances your math is indicating. Or at least not enough to notice. And this wasn't some small wing design with a high wing loading. It was a 14 oz 36 inch span wing that flew slow enough in the glide that I once managed to extend my post engine shutoff glide by a few minutes by riding out a passing strong thermal.

Even my free flight models, for the most part, have dealt with bad turbulence easily other than some that got caught in bad rotors near the ground that did not leave enough height for the model to recover from the disturbance. But once up over 30 to 40 feet they ride out and stabilize themselves easily in some really harsh turbulence that is often found in connection with very strong thermals. I just had to run a lot further to get them back....

So I'd have to suggest that you've got some nice numbers there that indicate that a lighter model will get kicked around more. But I have not noticed the sort of pitch related issue that your math predicts. I suspect you're missing one or more aspects of the real world conditions. Or perhaps the magnitude of this effect isn't that big a deal when taken in combination with the rest of the factors the models are being subjected to at the same time. Or perhaps it's less of an issue with models that have stability margins that are close to nuetral. Certainly the airplanes that are more like trainers with generous pitch stability were more work to fly in bad turbulence. But I always attributed the issue to the softer control response of a trainer not being able to counter the disturbance as easily.