I’ve used the new JP system in my GMR-15 for the past 2 weeks now. It seems like it cycles faster to me, but it may just be psychological. Recoil seems low, but I can’t say if it is lower than with the stock. Definitely a bit more painful to not be able to lock the bolt back to flag/reflag for the make ready. I forgot to charge once before starting which was somewhat embarrassing, but I guess it’s a learning curve...beep...click...huh?...Dang...charge...bang. My wife kept hers stock, didn’t want to fix something that wasn’t broken, and she’s still beating me for the most part, but it’s probably me and NOT the gun.
TL:DR The JP kit in the GMR-15 is nice. I do honestly think it’s better than the stock, but maybe marginally so. That said, this is a game of tenths of a second, so it probably does make a difference in the end. The question is if it is worth the $$ to you, and that’s a different question for each person.