Some observations and opinion(s)- After the "many" years of participation at Pearl,Jackson,Des Moines and numerous years at Regionals, had never seen a gun disqualified for an altered or changed main spring screw in a distinguished revolver. Unless there is a hole in the grip to access to make a change in the tension it is normally covered up with the often used target/after market grips, therefore the head not visible. Only other situation would be a if it was a grip that was not of the target style were it would be visible, but even then never saw one dq'd if it picked up the 2 1/2 lb weight when checked. Never heard nor saw another competitor file a protest on something anywhere close to this. I am referring to revolver only now. With a semi this would not even apply that I am aware of With semis, other ways of causing light pulls that will not qualify them for the distinguish match.
Now when they check the gun and triggers, I have seen them dq them for alterations to the trigger, push off,sights,alterations, to hammers adding trigger stops or any other visible alteration to a factory gun. I even saw them try to weigh factory "double action" only 92D Berettas-duh.
In many cases the trigger will not lift the 2 1/2 lbs because some wanna be smith got into the inners and changed the angle of the single action sear or the trigger nose along with other "lightening" and "smoothing" butchering. Absolutely, shortening the strain screw and/or backing it out will lighten the pull and as pointed out very possibly cause misfires with lighter hits. Thinning the main spring or over polishing, or bending too much can lighten and cause problems. "I" had never seen anyone pull the grips off to check a strain screw or main spring, not saying wasn't done.
There are things that can be done by a gunsmith that knows what they are doing to smooth the action up giving the distinct perception of a "lighter" pull and not actually lighten the trigger weight or cause problems.
It pretty much all boils down to the ability of the shooter and applications to the basics more the mechanics of the gun. Sure a smoother, finer action/trigger will help for sure and everyone always looking for that edge and a better mousetrap.