I don't think phase shift is that much of an issue, 90 deg out of phase can be cured simply by adjusting tube length, and I haven't seen any high end CIEM that had higher order xo. The problem with 3 way and above is their indefinite number of choices for cutoff frequency. And even with computer simulation, it's very unlikely the final result is in accordance to your intention, not to mention there are tube length/diameter and filter to take into account.
There's the Spiral Ear 5 way Reference solid silicone CIEM. 5 times cross over, iirc. The guy who makes them said he focuses on accurate attack and decay to achieve a detailed realistic sound. I think he may be onto something since the other headphone I know of with a very good attack and decay time is the SR-009, and we all know their reputation.
I was looking into buying a 5-way Ref however shipping complications out of the EU convinced me against it. Got a TG!334 instead. No need to change headphones if my ear canals change shape in a few years this way. I wonder how long TG!334 will last for... their build quality is very decent but my experience with all products is that everything is built to fail down the track. Planned obsolescence. Thank you cyclic consumer capitalism. Heard there was a model of Mercedes made in the 70's which could get to 1 million km's before needing a service. Mercedes thought they could make more if people bought more parts and cars more often, so models after that were designed to need servicing a lot sooner or else they'd break down.