Larry;
To borrow from Jim, "Well Said"
To comment on this statement "although in Burrard's case in particular, he can be misquoted on the subject if you do not read ALL of what he said." This is very true. It is in fact "Fact" that both Thomas & Bell were guilty of this, taking his "Warning Against" using the higher pressure "True 2 3/4" shells in guns built for the normal 2½" load as complete condemnation of hulls longer than the chamber. He did of course also fully cover the putting of the 2½" load in the longer hull with no undesirable effects.
It might also be mentioned that Bell did in fact fire some "Low Pressure" loads put up in 3" cases in 2½ chambers which stayed within the pressure range for the gun. He "Did Not" advise doing this purposely but checked it out just to see what would occur if it happened accidently. Note this was not a factory loaded shell as there are, as mentioned, no factory loaded low pressure 3" shells.
Personally I am not too concerned about the fired case lapping into the cone a little, but would never intentionally fire a shell in which the loaded case entered the cone (likely to occur with the 3" hull in 2½" chamber. The pressure begins to fall virtually with the first movement of the charge, so long as the crimp is not impeded in it's initial opening, pressures would seem to be not much affected.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra