I don't have time to search at moment, but I know I posted (somewhere in AL group) a link to comparison of bunch of calibrated microphones. From perspective of frequency response, they all performed pretty much the same. . I switched from a umik-1 to isemcon but it was only because of the timing issues I experienced measurement to measurement. Some people don't seem to have the issue.
No way to prove it, but I believe if timing wasn't an issue, my results would not been any different between the two microphones.
My umik-1 is also custom calibrated, and for me that also made a difference. My umik-1 did not fit the "average" calibration data supplied for the batch.
Comparing the custom to stock calibration files, I was surprised with some of the differences as they were significant i.e. not just a couple db here and there.