Okay, now I'll have to ask: what is your definition of handle? Because technically almost any iem with a semblance of R&D can handle the poorly mastered tracks, but only the few can handle the gloriously mastered ones and bring them justice.
As for how listenable, it'll come down to IMO mainly two things: your tolerance for siblance/harshness and your love for the music. I find that the worst consequence of bad mastering is siblance and harshness in the ranges, but if it is tolerable then it'll be fine for you. This issue is worse in IEMs like the Zeus because they're brightly tuned, while less of an issue on the also brilliant Aether and harmony 8.2, which are dark sounding in comparison and won't reveal much of the harshness. So I'd suggest finding a iem with emphasized treble like the 8.2, maybe the Aether, maybe the SE5U (from my admittedly limited experience), which will all vbe more listenable than something like the Zeus, Fourte, A18, etc. the strength of the bright IEMs IMO is that it you feed them the right material, they will absolutely SING compared to the darker sounding IEMs.
EDIT: I also have to point out: rig synergy. For example: Please please please don't get a really bright source like a 240SS and pair it with a silver cable and a Zeus. You're asking for trouble. Nobody likes to, and ideally we shouldn't have to, but you have to know what gear you're working with well enough to decide what to pair them with.
The love for music? Its very simple. If you love the music enough, if you feel it deep in your bones, you won't give a **** whether it is harsh, siblant, lacking dynamics, etc.
I'm often teased for having so much TOTL gear yet walk around with my iPhone 6 and one of my many pairs of Apple Earpods (which aren't bad I might add?!?!?!). I simply just enjoy the music enough that I listen to the music, not the technicalities.