I recently performed an enhancement on my Browning Hi Power using a kit from BHS Spring Solutions. I was amazed how much of an improvement it made. I often shoot my 659 along with the BHP and quickly realized how clunky the 659 is by comparison.
I realize I am comparing a DA/SA to a SA only but hope some of you folks have experience with helping the 659.
If you detail-strip the 659, and polish the parts that rub on each other, changing nothing else, your felt trigger will dramatically improve. If you then change out the mainspring with lighter ones it will improve some more.
Some will tell you not to alter the weight of the mainspring on a defensive weapon, but I trust my life to many guns who's springs are different from their factory originals.