IMO, the compensted nut doesn't do anything noticable. I have 2 JP's, one with and the other without, and they both play/sound the same to me. The compenstaed nut only really comes into play with open strings anyway.
It would probably be just as easy to put an EBMM compensated nut on as it would a Buzz Feiten, though I don't think you can get the EBMM's. Both will need cutting of the fretboard for installation closer to the bridge. The difference is the EBMM does not need the tuning offsets. I have tried the Buzz Feiten and the EBMM compensated nuts, and the EBMM sounds better to me and is much easier to use.
And the compensated nuts do make a difference in tuning at all playing positions on the board. Barr chords for example all play sweet in tune no matter what the fingering or position with the compensated nut. I have one MM with and one without compensated nut. The one with a compensated nut is a big difference as far as playing in tune all over the neck. But I still like the other one too. Sometimes a little sour sounds good in rock.
Can one buy those compensated nuts from EBMM? As with you, I have my JP7 with the compensated and my JP6 without the compensated. I'd like upgrade my 6 to the same setup if possible.
I think the "sweetened" tunings on the Peterson tuners are an alternative to compensated nuts. Of course, I could be wrong. I intonated my guitars with the Strobosoft and then used a sweetened tuning. Chording throughout the fretboard sounds noticeably better.
I have not tried any Feiten equipped guitars but the idea of compensating the nut would be a godsend to me. On a standard guitar, tune a barre chord perfectly and then try a few open chords - I hear those bad notes and they drive me crazy when recording!
Of course, perfection is not really perfect anyway - in equal tempered tuning the only perfect interval is the octave, all other intervals are a little sharp or flat compared to what the ear would really like. On a well tuned guitar try an E-shape F# Major and then compare to an E-shape G minor. It's the same Bb/A# but it sounds slightly flat for the minor chord and slightly sharp for the major. Not compensating the open strings just puts you further away from that 'ideal'. Tune the open strings properly with a tuner and compare an open E major and an first position F minor. If the nut is cut too high, it gets even worse!
Of course some musicians don't notice, many don't care and the audience probably won't notice at all but a well fitted compensated nut would make MY ears a lot happier.