Eh, 2520?
Sure some mods would be needed to make a Favorite style .25-20, but if Stevens could do it, Varner can do it. The Stevens Model 44 (NOT 44 1/2) is nothing but a scaled up Favorite and it took stuff as big as .32-40 and .38-55
DeHaas covered what it would take to make the Favorite much stronger years ago; steel frame, threaded barrel, tighter fit of breechblock to frame, and a bit heavier linkage.
Like I said, I only heard a rumor. Could not get a response from Varner, so I rooted around to see if it was feasible. Looks very feasible for .22 Hornet, .25-20, .32 H&R, etc.. I'll agree with you that .256 Mag would be a bit too much.