2-piper the forcing cone may not even the highest area but it is a very close second and the removal of any metal in this area is uncalled for in my opinion. And I have see a few guns that had "forcing cones" elongated to remove deep pitting. How much metal did they loose? Again I question why it was done as much as it being done in the first place. And I would be very unlikely to buy any gun which has had its chambers reamed from 2 1/2 or 2 5/8 to 2 3/4 or worse 3". I want as much metal as I can get four or five inches from my fat nose. You are right it is almost certainly safe if done properly but I contend it is almost as certain to be an unneeded alteration.

The improvement in .410 is most likely due to less barrel scrub of the shot as it enters the bore. If 25% of the shot rubs against the bore in a .410 only 5-7% does in a 12. So you'd expect more improvement in a small bore over a large bore.

I've seen a lot of trap shooters have forcing cones lengthened to 4+", bores over bored and ported. All in the name of progress and looking for that extra edge. Nothing beats ability and practice. It all come down to the "barrel experts" convincing people to get work done so they can make a living too often.