The heavier ammo (115gr) tends to be more problematic in the P-64 than most of the standard 95gr fodder. You should definitely take a trip to the P-64 forum http://www.p64resource.com/forum/
for specifics ... lots of info over there. If you go with anything lighter than a 20# mainspring, I would make sure to test it thoroughly as I have seen spontaneous magazine ejection occur as the mag gets close to empty (the mainspring does double duty in the P-64 ... hammer and mag release). I'd second the suggestion to put the heaviest recoil spring that you can fit into it ... the "snap" from the slide impacting the frame almost goes away with the heaviest one from Wolff (22# IIRC).
Great little pistol!