1 - check the difference in sustain with the bolt on without the guitars plugged in
2 - work on your vibrato, this is a great "sustain enhacing device" and it's FREE, I can get my guitars to sustain quite long unplugged - actually, if you have an acoustic, practice on THAT, if you can make that sustain, you can make any electric sustain
when plugged in first try an OD pedal, my BOSS SD-1 will make my strat sustain almost forever - and that's low volume, just some OD and a touch of vibrato, a LP design should do even better, as they are supposed to sustain longer just because of body mass.
if you're satisfied with your tone, I wouldn't start putting new PU's in - your tone WILL change - if you're not satisfied, change them for PU's that sound "better" to your ear, just don't expect magical improved sustain, IT WON'T HAPPEN