[jeffbee]

Kind of an odd undertaking, though. The biggest improvement you can get with IEMs by far is custom fitting. You could take one of those middle-of-the-pack IEMs from that big list, spend a few hundred on fitted tips, and get a better result than the non-custom stuff at the top of the list.

[slapo]

What's this based on? AFAIK, custom molded tips, should create a better seal than generic ear tips, but unless they're molded with optimal insertion depth for a given in-ear, they might change the perceived frequency response. Bearing in mind that the material of the tip also matters, one might not necessarily get an "improvement", but merely a change in sound signature and a more secure fit.