My experience with multi-armature driver IEMs + impedance adapter is negative (UM2, E5, SF5P) for most the part, but I did not try it out with the E500/SE530.
With those 3, it screws the sound, likely due to the crossover implementation.
From what I understand, the SE420 uses a pre-emptive crossover, whereas the E5 (and presumably the SF5P and UM2) uses a co-operative crossover implementation, which may contribute it to acting like a single armature driver (those that I tried/used) when it comes to an impedance adapter implementation.
As for the improvements, I just feel that the SE420 is better with it, being tighter, extended sparkling treble that does not get masked by its initial very present (yet ironically unhumped) midbass, a more smoother midrange as opposed to a somewhat diffused one. It basically feels like a Shure E4 done right in every single way possible.
Clannad in Consert and Eire: Island of Saints sounds astonishingly jaw-dropping on it (the details, the tonality and instrumental timbre), surpassing the RP-21 by quite a fair amount IMO in all aspects (not huge but still rather noticeable) with the exception of midrange, where the RP-21 gets slaughtered IMO (The RP-21 does have more kick to the bass though, so I still prefer it for Rock and Video Game OSTs, and the RP-21 is more forgiving in general).
Presently listening to Chet Baker in FLAC while I'm fleshing out my storyboards