IMHO, planar IEMs if they do need any help, it is for a DD to help their low end bass excursion, which is a weakness in some planar IEMs (note this does not apply to planar headphones but IEMs, probably because of the smaller drivers used in IEMs).
Most planar IEMs already have good treble extension and resolution, and I think the added BAs don't really bring much value add to the table. In fact, if you see the DUNU Talos, which has a hybrid mode with the BA kicking in, it makes the Talos very shrill and harsh, and most users keep to the pure planar switch on the Talos.