I run mine at 6 lbs to keep some consistency with my heavier 617. Same rebound spring my 617 has puts my 625/627's at ~4 1/2 lbs. Radically lightened hammers are necessary. Shortened firing pin spring (on FMFP guns). Rebound springs get replaced. Some replace main springs some bend/recontour the stock spring. I modify Wolff reduced power main springs. Definitely requires some relearning with really light DA pulls. That's why mine stay 6ish+ lbs.
Randy Lee
Apex Tactical Specialties has them running reliabley under 3 lbs.
More good info on this type thing can be found here:
Brian Enos's Forums... Maku mozo! -> Revolver Forum