Rsq, I too approached the idea of a square bottomed stop with a lot of questions even though I had read a lot by folks here and on other sites who said it was a worthwhile addition.
I began with my Officers model, where I thought it might do the most good, or at least show the most significant results. Although some suggested rounding off the bottom ever so slightly, I decided to try it out with a perfectly square edge. After fitting it to the slot, I loaded up some light target loads consisting of 4 grains of accurate #2 behind 200 gr lead semi wad cutters. Because of what I had heard about the modification, I wasn't certain that the pistol would function correctly because of the light load, but I also didn't want to damage anything.
I noticed two things immediately.
First, the oversized stop held the extractor correctly and my case ejection was much more uniform. The brass cleared the ejection port cleanly but instead of the cases going 10-12 feet from the weapon, they landed about 6 feet and to my right rear. No brass marks on the slide-this pistol has a slightly lowered ejection port compared to the GI issue and the lower front edge is radiused to clear loaded cartridges from the chamber.
The second thing was recoil. I cannot say that it lessened the kick but it felt different-more of a push back into my hand and less muzzle rise than with the standard firing pin stop, or so it seemed to me. To test what I thought was going on, I loaded some full power 200 gr JHPs to see if it was a fluke. Of course, the recoil was more pronounced but here again I did notice less muzzle flip.
After a few hundred rounds through this pistol, I decided to try it out on my government model with similar result. It functions with any reasonable load that I have tried. Although I have less than 2000 rounds with this modification on this particular pistol, one additional benefit I have noticed so far is that the spur hammer has not bounced off the standard grip safety, or at least not enough to show any marks on it. I have an older government model which shows a very slight wear on top of the grip safety from the hammer touching it in recoil.
When you finish your testing, I would like to hear of your personal experience!