Apart from the markedly higher self-noise of the tiny-tiny electret capsules, on a fundamental level I'm not sure that fully omnidirectional response at high frequencies is a thing to want. When I listen to recordings that I've made with normal-size omnis (and I've made many over the years, with AKG, Neumann, Schoeps and Beyer microphones among others), my regrets are never, "Gee, if only my microphones had picked up more energy above 6 - 8 kHz from the sides and rear."
I think we really do want the high frequencies to be attenuated off axis. Omnis have a hard enough time creating a stable stereo image without letting unattenuated high frequencies from all angles into the mix.
All the usual techniques of recording stereo with omnidirectional microphones are based on decades-long experience with the characteristics of conventional-sized (e.g. 20 mm diameter) pressure transducers or larger--some of the classic stereo pickup techniques such as the "Decca Tree" are based on conventional omni capsules that are made "acoustically larger" by embedding them in a 40 mm sphere (although the shape is as important as the size in that approach).