Reducing a GP100 SA pull to 2.5 lbs. is almost impossible and be 100% reliable with all factory ammo IMO. I think many exaggerate what light weight they think they need. I am totally opposed to light weight springs IF the gun is to be used in any shape or form for defense. I think as well that many believe a hair trigger will automatically make them a champion shooter. It will not any more than owning a Steinway will make you ready for Carnegie Hall.
I would recommend an action job by a 'smith familiar with Rugers as, despite what many Colt & S&W snobs believe, a Ruger can be tweaked just as good IMO (and I own those aforementioned as well). The secret is too make the pull smoother and not necessarily lighter. Finally, shooting a DA revolver in DAO mode can make one a far better shot than merely lobbing lead downrange SA.