I'm not expert and wondered the same thing as you - until I replaced the first one which by the way is really easy. I replaced the J-lock on a LVSF I had, I used the Gre-tan bolt shroud/firing pin assembly because it matches the finish a little better than say the Callahan.
Anyway, as far as could I feel anything different, maybe a little, seems the lock-time was a little shorter, can't say it made me shoot any better. But once apart, the factory spring really is a ugly looking unit, I think someone has posted pictures - it really is a twisted mess of a thing, the Gre-tan or Callahan look much cleaner. If going for lightweight, you also can shave off about 2oz with a after market so for me it was worth the $50 or so it cost.
