Larry;
The info I received from you on this was essentially useless. Reason "NO" data was included as to what the test loads themselves consisted of. No mention as to what type of powders were tested. As you have never given any particulars I am asuming you have none. Since Thomas did not state any I also assume either he did not have them, OR didn't want to include them. Now it is a well known & established "Fact", Measurably So, that the progression of shotshells from Black, to Bulk Smokeless, to Dense Smokeless have each resulted in a reduction of Recoil. A 3 Dram charge of BP weighs 82 grains. A 3 Dram "Measure" of bulk Smokeless such as Schultz or DuPont Shotgun weighs 42 grains. A charge of Dense Smokeless equivelent to 3 Drams of BP such as Sporting Ballistite or Infallible weighed 24 grains. These differences in weights are significant enough to make a discernable difference in recoil. Thomas' reasoning on this subject, IE, that the fast powder loads had less felt recoil because It occured so fast the shooter did not have time to think about it, while with the slower powder it was of longer duration so the shooter noticed it more, is about as "Patently Absurd" as saying it's OK to lay your thumb atop the nail head as an aiming point "IF" you "Hit it Fast Enough".
As it stands right now it seems virtually everyone with an opinion on this subject, Except; You, Mr Legg & Myself think one can go to a slower powder & reduce felt recoil. You on the other hand, in your blind devotion to the words of Thomas, without in the words of Mr Bell "Finding Out for Yourself" apparently think one can switch to a faster powder & reduce felt recoil (If you didn't say that, you have said Nothing).
Mr Legg & Myself take the opinion that unless either total weight of ejecta or speed of that ejection takes place no significant change in recoil will be felt.
I Guess everyone will just have to believe whatever they choose to.
To answer the original question;
IF the longer shell were loaded with a load appropriate to the gun & if the loaded shell had clearence past its crimped end in the chamber, I would not concern myself that the end of the fired hull, after the opening of the crimp, lapped up into the cone a bit. I would expect no significant change in either pressure, velocity or recoil. If the length of the loaded shell were enough longer than the chamber that it protruded into the cone before firing, I would refrain from firing it in that gun.

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