From those dubious "official" frequency response plots, it seems like the biggest EQ differences between a U67 and U87Ai would be that the U67 tones down the 10KHz resonance of the capsule slightly more (so it's darker by a dB or two) but the u67 rolls off the bass more, with a MUCH higher corner frequency. (They show the U87ai as flat down to about 70 Hz, but the U67 bass rolloff starts well over an octave higher.)
If that's even roughly right, it seems like the biggest sonic difference between the two would very likely be the overall FR curve (assuming you're not driving either circuit really hard and getting into serious distortion). They use the same headbasket, right?
Has anyone ever seen a blind test where the two are EQ'd to match in those obvious ways and you can listen for the remaining differences?
(For that matter, has anyone ever seen a blind test or just a shootout of any two mics where the overall FR curve is controlled for? I haven't run into them. [Edited to add: aside from Julian Krause's YouTube videos EQing an SM57 and a Behringer XM8500 to sound more like an SM7b, but that wasn't much of a test.])