I have C421 (OPA2227) with me, I'm really liking the sound it's giving me with my headphones. It seems to lean a touch cooler in tonality and clarity, sounds clearer than C5 in comparison, soundstage seems wider. However it doesn't seem to have quite the same (close) refinement/control as C5 though overall I like C421. Until....I plug my 10ohm Dunu DN-1000 hybrids into the amp. The high 10ohm (roughly) output impedance of C421 caused some rather obvious problems. My Dunu sounded hollow like someone sucked the mid range out or like the singer was coming through a tunnel. Go back to my fullsize headphones I really enjoy it.
So I guess this will depend on a few factors which I decide. I think what C421 does to my headphones is worth the purchase as it sounds great, however losing the ability to pair with a majority of my IEM collection kind of takes away the glory. I'll pair C421 up with ER4S this afternoon once it's charged again and consult this further.
So in a nut shell C421 great with my full-size headphones better than C5 to my ears minus some refinement but the high output impedance is really a bummer.
Perhaps try it on a full charge (if that matters). The C421 2227 was my "go-to" for IEM use for a number of months..