I think if such an IEM existed, we'd all own it, and do away with the need for the electrostatic amp
I'll probably be lynched for saying this, but with silicone tips on the KSE1500, the IEM I've measured with the closest frequency response is probably the Sennheiser IE800. This has a bit of a nasty treble peak at about 11 kHz though, which you can't do much about because those eartips are proprietary. With the KSE1500, you can tame the treble with Comply foam or Shure olives. I personally can't take the IE800 for long periods - it's too fatiguing.
The FLC8S is a bargain for its price, and you can tune its FR to your liking. For me, it can get quite close to the KSE1500 in terms of being an enjoyable listen.